Well I would begin my figuring with the tried and true 10 feet of .028 for a 150w load. Garry at xs says to shorten by one foot for 15w extra load. I think that is a good aproximation for small changes, but by that rule if you want a 300w load you have no cap tube at all. Flow rate is inversely proportional to length meaning 5 feet should give you a 300w load 2.5 feet should give you a 600w load while 20 feet should give you 75w load.
I'm rambling. Here is what you need:
Length (feet) = 1500/Load (watts)
So in your case 1500/250 = 6 feet.
(for .028 cap tube)
If you want more length, go with .031 cap tube and 9 feet 7 inches will do you fine.
(known: 1.6 feet of .031 cap tube = 1 foot of .028 cap tube)
Keep in mind that I ran, and Garry assumes you run your cap tube in your suction line or around your suction line to get a some subcooling of the incoming flow.
I dont think cap tube length is dependant on coolant temp or whether its a chiller or DD. All that matters is load.
Evap temp doesn't matter because the only time it would is if you are supperheating the refrigerant in the evap. That will only happen if the vaporization of the flow of liquid refrigerant is not sufficient to absorb all the heat from the cpu at that temp/pressure and therefore some of the cpu heat heats the refrigerant above the evap temp. If you guess the cap tube length right, this shouldn't happen.
Edit:
Doh: you beat me matt
