Well, you can estimate an approximate level by looking at the bar. 6000 heat means 40% durability cooling elements, which is slightly below half durability. I mean, it's imprecise, but +/- 500 heat doesn't really matter when you're aiming for 6000/10000 heat.
Also, to make sure you get the 40% right, you can simply take a tool with a known amount of uses (like a drill with 150 uses) and use up exactly 60% of its energy reserve. For a drill, that would be 90 uses. The remaining bar is what you should compare the state of your cooling elements to.
Dont estimate > Simulate!
Know , dont guess :
you always start with heat=0
if you make a reactor where all the cooling elements are connected by DC's
then external cooling + internal cooling - heat produced = the correct number and if it equals 0 then you have a perfect breeder.
(as long as the reactor contains a depleted isotope ofc)
next all you have to do is (manually) adjust the heat (which will remain stable) with lava and water and this is why I always have atleast 1 open slot in all my breeders.
also is the heat/efficiency limit still at 9000?
I think we should have an alternate efficiency classification system for breeders as no well designed breeder reactor should have more than E for energy generation
I have come up with two systems
1: we measure it in uranium bars used per depleted isotope enriched;
at default with 0 heat 1 bar and 1 isotope it is 1/8 * a 1 bar 2 isotope is 1/4 and a 2 bar 1 isotope is 1/8 because it takes half the time but uses twice the bars.
as we now know for each 3000 heat the speed of breeding doubles so example #2 would be 1/2 at 3000 heat this means that the guide stating:
"8tuple your urainium yeild" is only true under certain conditions if you use #1 you are simply wasting time at #2 you are doubling your yeild #4 would quadruple your yeild and my optimum design breeder (the one with the bucket in it) would give 16-2=14 times the normal yeild (2 are used in the process). think this through and correct me if I am wrong 
I also sense an imminent rage/spam/generalnoise on the minecraft forums tomorrow (depending on your timezone ofc)
including great stress on modders as the engine (i have heard) has been changed significantly
good luck with that XD
