To add to the good info that Joe has already given....
Make sure that you follow the paint mfg's recommendations on thier paint data sheet...if the customer needs "12 mils of coverage"....don't just put it all on in one coat without looking at the data sheets....the data sheets may not allow you to coat it that thick in one coat...you may need to apply two or three coats to achieve the final coating DFT that the customer requires....as Joe stated, apply too much paint in one coat and it will mud crack.
SSPC-PA2
5 spot measurements (average of three readings) for each 100 square feet measured. Note that individual readings are not subjected to rules but are included in the average for a spot measurement. No single spot measurement can be less than 80% of the specified thickness and no more than 120%.....additional measurements may be necessary to define any non-conforming area. Mfg data sheets can be referred to determine if a higher maximum thickness reading can be allowed under certain circumstances.
Another standard that is often referenced is ASTM D 1186 and has two methods of calibration to watch out for method A and method B.
Make sure that you follow which ever standard is called out...if no standard is referenced, I'd go with SSPC-PA2.
edit:
Here is an example of how to document the DFT measurements and the averaging...