What on earth are you basing that on? A 60/40 split is extremely unlikely given how much more air the back fan would move at similar power and noise levels. It wouldn't be as simple as stopping one fan and measuring GPU temperature, as that is not going to be the sole source of heat on the card. That, and even if you stop the front fan you might very well end up with a situation where the die remains cool connected to the back fin array with massive heatpipes, but the VRMs that rely on the front fan for cooling begin to overheat and throttle. If you really want to see how much each fan is exhausting in normal conditions, duct both IO and back exhaust through air velocity meters to measure CFM, and monitor intake and output temperatures to calculate the actual heat dissipated.Of course the axial "blower" has to dissipate quite a lot of air and heat. The card would probably burn out, at least throttle very hard of one functioning fan only. Best bet is that at least 40% heat is going out from the back. Would be easy enough to stop one fan at a time and observe throttling/ high temps.