I don't think you can really associate it with a percentage. I think the right thermogenics are more about quality over quantity.
I've always been fairly lean. Off-season I teeter around 9-10% body fat. This is just where I am healthiest (good test levels, immuno function, etc.). I've been around this body fat for near-forever so its no shock that this is where my body is most comfortable.
When I start getting in-season that has to come down around 5%. I can get down there without supplements, but its a long tough road often flirting with overtraining. I notice when I take HOT-ROX my weight loss is quality weight loss and I don't get the feeling like overtraining is just waiting to catch me napping.
Also, HOT-ROX has always helped me keep (if not improve) my LBM and performance levels throughout a taper. This is extremely important if you are expecting to keep this off. I generally have to keep the body-fat under 6 for 4+ months and still be able to perform my best.