I don't think it's hard to keep a large frame balanced between FC and trail. GrayJay pointed this out well. It gets hard when you're building small frames, or frames for people who have limited reach. Once you get below a FC of about 585 you suddenly have toe overlap becoming an issue. If you slacken the head tube and add rake there is more of a tendency for wheel flop, and added rake might lower the trail more than you'd like. Small bikes hit all the edges of normal. In the frames I've built for myself to ride, I've found a FC of 590 to be ideal for my size. With adequate FC the range of trail is less important. I don't know, just seems to work for me, but it's a holistic thing. People talk about how great a short wheelbase/FC frame handles, but to me good handling is confidence at speed, and I've never had trouble getting around corners. Probably the most important factor in all of this is the rider and his/her ability to be fit for the bike.