Once I find good relative boat speed to other boats I mark the sheets. Say I did this in 12 mph winds. So, in winds of that speed I pretty much know where to set the sail. However, say the wind diminishes to around 5. Then I ease the main sheetand note where my mark is. If the wind increases, I sheet in tighter and again note were the mark is.
It is not an exact science, but it is better than guessing each time you sheet in.
Rick