At what weight (in terms of % of max weight or min weight) do true fluid heads perform best at? Does this depend on whether what is mounted (in my case, a DSLR rig) is front-heavy?

I think there's factors other than weight to consider first. You want to first balance your camera on the head so that the camera's weight is distributed equally (front and back) on the head's counterbalance mechanism. The amount of drag you use is another factor. Then add to all that differences in the the head ...

If you are shopping for a head for a front heavy DSLR, make sure you get a design with a sliding QR plate so you can balance the load properly. Adjustable counterbalance will fine tune the load on the head when you are not exactly shoot level. Adjustable drag gives you something to push against. I would expect all of these come into play before dealing with the weight range of the head.