There is only 3 ways to compensate for decrease light on the camera----lower your f-stop, lower you shutter speed, or increase your film speed.

Also--the higher the magnification you are shooting the pic at--the less light gathering capabilities the lense has---so if you shoot at lower magnifications it requires less light.

Sounds like your only options would be to go up to 800 or 1000 speed film.

good luck
todd