I can switch the offending code to use floats, but as you can see, the rotation slows down somewhat. Regardless, the z88dk maths libraries are doing a sterling job there. That's done partly using floats, and sin and cos from maths.h. It'll be much faster once I've given it the asm treatment.
Okay folks, there was a quick fix - use a long in the perspective calculation and leave the rotation as int.
Working on high resolution typesetter code (up to 3600 pixels per inch, up to 36"), coding on a 68020, we used 24.8 fixed point as our main representation. It's really hard to keep integer maths pixel accurate, but when you've got 8 spare bits, life get easier. (this was 1986/87)