laser level

Borrow, buy, or rent a laser level that will allow you to mark a level reference line all the way around the space. You will need to adjust benchwork legs to suit, assuming you want the benchwork to be be level. Then, keep a long bubble level handy as you put in track.

Operated a couple of times on a 6'5" guy's layout he had built to be a comfortable height for him, in his garage.  Ended up building several walking platforms to accommodate his average-size operators. The tracks on the back wall weren't too bad, but they were pretty high out toward the garage doors.

I wouldn't cut the legs

I wouldn't cut the legs exactly,

Build your frame,and prop it up level with whatever you got handy. Then mount the legs,and remove props.

Plan B; there are many ways to make adjustable feet, and more then a few you can buy.

Plan C; build the frame, and shim under each leg

Small Angle

Six inches over 22 feet is slightly more than 1 degree. Across the 3.5 inch width of a 2 by 4 would be a difference of less than 0.08 inches. Even less if they are turned so the slope is across the shorter dimension. I don't think that I would worry about that on each leg.

Don

"The garage has a sloping

That's a pretty big difference from end to end so I'd pick a spot near the middle of the wall and figure out how tall you ideally want the layout to be then level both ways from there so half the layout will be a bit taller than ideal and half will be a bit lower. Once you know the correct leg length you can make longer and shorter legs to suit their location along the sloping grade. I wouldn't bother with angling the bottom ends of the legs to match the slope as mentioned above it's a very slight slope across the width of a leg.You could buy metal buttons to drive in the leg bottoms to let them rest solidly on the slight slope.......DaveB 

All in the way you look at

All in the way you look at it. That sounds like a lot and I guess technically it is but my math says it works out to slightly less that a 1/4 inch to a foot. I really don't think cutting the legs to any sort of slope is necessary and my opinion is that one could handle it with ordinary leg levelers based around T-nuts and bolts. Drlll the appropriate sized hole in the bottom of the leg, hammer in a T-nut and then screw the bolt all the way in. Build your benchwork with the pre prepared legs and then using a good level, go back with a wrench and level as needed. That will also allow adjustment for further settling or whatever has caused the uneven floor surface. Caution is warranted because the slightest unwanted slope can wreak havoc with free rolling wheelsets but don't overthink it. Maybe you could cut the legs on one end slightly longer but I think the levelers are a good idea either way.

Almost all of the solutions

Almost all of the solutions are already mentioned. No bother with the slope at the bottom of the legs, it is negligible. Best solution for me seems the legs with equal lengths and with adjustable ends(threaded rod in threaded nut in wood). The adjustable legs are necessary in any case, even if you have a "perfect" leveled floor. An exception is when you build from the floor to the top and the legs are fixed on it. Then you have put in level the upper horizontal beam. Fix your layout horizontally on the walls to avoid loading the legs horizontaly.I miss the details of your structure. 
You can build the frames with the legs (long direction) on the floor and then rise them to the final position. Level the frame by adjusting the legs at the ends and then, I suppose you have two parallel frames, connect the two frames. You can do this alone, but with help it is better and the beer tastes better.

Save yourself from a lot of work and expense....

if the black horse can’t deal with the grades you should consider a logging railroad with Climaxes and Heislers!!!

( ;> ) )

Nick

focus on a level reference line

since you already know there's a 6" drop, presumably you already have a level reference line on the walls.     If it's only approximate and don't have access to a laser level, you might use a garden hose level to put a reference line on all the walls..

but as others have said, since the angle < 2 deg, you don't need to worry about cutting the bottom of the leg at an angle, it's 0.08" over 3.5" (2.3% grade)

obviously a 6" difference means the leg lengths will vary.  those reference height lines may need to be transferred to the legs to determine the layout height.     

I think Michael T makes a good point, i've had cars roll on what looks like a level grade..  Wood expands with humidity and may require seasonal adjustments.   Not sure if shims are enough or if leg levelers are needed to be safe.

4x12 on a sloping floor

You say a 4x12 layout in that space - is that free standing, or attached to the wall?  I’d guess free standing, or you’ll have access troubles (4 feet is a long reach).  One thing to consider is that the floor is almost certainly not uniformly sloping - it’s quite likely that if you cut the legs to perfectly level the surface, then move the table a couple inches one way or the other, it will wobble.  So you’ll need adjustable legs.  Ideally a good deal of adjustment.

Working on my garage layout too

I'm skipping the legs altogether where possible.  I started to find the mid point of the slope in the approximate  middle of the slope in the area while the layout be and marked a spot wall.  This will give me an average layout height when finished of 52".  It will a little more as the operator heads left and little less as they head right but the track will be level.   I established a level line around the whole room.  While I would recommend a laser level I did it the old fashioned way with pencil and a 6' level. 

PM me if you have any questions.

When legs are required I will work off that one middle point and extend from the off layout attached to the walls.  The frame work will be mocked up and, working with the level, establish each leg as required in the exact point it is needed.  Each leg will also have an adjustable foot to compensate for any irregularities in the floor and allow me to adjust for best performance.

Steve