A/C Leaks

Common problem. A/C not blowing cold enough. Re-charge, put dye in it - check for leaks. My techs tell me that in 90%+ of the cases, you won't immediately find a leak because it takes a while for the dye to work through the system and so you have to have them come back at a later date. This brings up a number of questions:
1) Do you charge diag the 1st visit, the 2nd visit or both?
2) Do you charge for full capacity of freon that you evac and re-charge once or both times?
3) If they have a service contract, you can't successfully file the 1st visit claim because you don't have a "failed part." How do you handle this?
4) Do you agree that 90% of the time, you can't find the leak on the initial visit? If not, what is the secret to finding the leak?
1) Do you charge diag the 1st visit, the 2nd visit or both?
2) Do you charge for full capacity of freon that you evac and re-charge once or both times?
3) If they have a service contract, you can't successfully file the 1st visit claim because you don't have a "failed part." How do you handle this?
4) Do you agree that 90% of the time, you can't find the leak on the initial visit? If not, what is the secret to finding the leak?