There are formulas for TECs. You need to know:
1) TEC model
2) input voltage on the TEC
3) hot side temperature (how well can you cool the backside of the TEC)
4) wattage of heat input on the cold side
In fact, go to melcor's website. They used to have a free program to design a system!
Now, the kicker is that it is hard for us watercoolers to know 3 or 4. The hotside temperature really depends upon how good your cooling system is. The wattage of heat input depends upon a value that the CPU manufacturers don't give out and it doesn't include the parasitic heat input caused by leakage (where heat moves in from the motherboard and surrounding environment to a cold CPU).
If you know the C/W of your waterblock and system, it is easy to know the backside temp of the TEC. I know very few estimates of C/W that are accurate, mostly because people's system's are never exactly the same (is your tubing length etc... the same as the reviewers that do system-styled reviews?). Maybe you can use something like 0.2 C/W for your system as an estimate with one degree of precision (yours may be 0.13 or 0.27 but make some good guess). Then use the wattage of your target TEC to calculate the backside temp.
If you perfectly insulate the CPU and TEC, you can assume that only CPU heat is a factor. Work from that as a maximum coldness you could ever reach.
Finally, you could just try it based upon the best suggestions from this forum because it is just too hard to really calculate the actual numbers that go into the design program.
