I have done Fifteen Inches

I have cut my own kerfs, and my biggest risk of breakage was carrying the strips to the layout room.

Keep the Homasote flat on the subroadbed and bend it slowly.

The limit of how sharp a curve you can get is not so much breaking the stuff, but the closing of the kerfs along the inner edge. I have used my saw to open up the kerfs into triangle shapes.   On the outer edge the kerfs open up, so you may feel the need to fill them with caulk or whatever else.

Good luck, and be careful breathing when you cut this stuff. It's a good job for outdoors.

DrJolS

Curving kerfed Homasote

The Homasote roadbed should be in two pieces.  One piece is kerfed from the bevel side in and the other from the square side in towards the bevel.  Both pieces should be bent so that the kerf is towards the inside of the curve.  The limit of the curve has to do with both the depth of the kerfs and how far apart they are.  I find about 2/3 of the way through about an inch apart will give you down to about a 17" radius curve.  They are delicate to handle and need to be supported when untaping or moving a single strip from storage to the layout.  Hope this helps.

Cutting homasote

I found some saber saw knife blades made for leather cutting, no dust when used!  Now I cut in the RR room.