Overzealous trim-sawing and the wrong kind of light have ruined more of my stones than any other cause. I've tried every kind of indoor lighting scheme I've been able to conjure up, including installing two quartz-halogen floodlights above my grinding unit. Being able to really see what you're doing is the single most important factor in cutting this stone. Maybe your eyes or techniques are better than mine, but I now cut all my fire agate in direct sunlight. I made a portable table with casters on the legs and on warm, sunny days when I have fire agate to cut I slather on sunscreen, roll the unit into the driveway and have at it.
Direct sunshine makes it possible to do some very accurate grinding.
It's the nature of most fire agate cabs to require some contouring to follow the color layer, and flat grinding wheels are designed to cut convex or flat surfaces. Unless you decide to invest in specialized diamond wheels shaped for this job, the only way I know to do it is to use the edge of the grinding wheel. You'll discover that, with care, the face of the wheel does all the work and the edge gets very little wear. This type of cutting can be dangerous. Work very cautiously, bearing in mind also that putting pressure on the edges of most standard diamond wheels can quickly ruin the wheel.