While everyone most likely has developed some type of "system" that work for them...
A basic system is evaluate the job, based upon what is to be painted
determine the SqFt of the painted areas
know how long it take to cut and roll those areas (spray etc)
allow time for set- up and prep add this in
determine the required Man Hours
multiply manhours by the going rate in your area per hour
add in fixed cost variables
this should provide the bid or quote price.