I don't mean to be a malcontent in this thread, but to me all this measuring is beyond anal and is totally unnecessary, but then again if it floats yer boat, go for it. The soundcheck is gonna be different every night, the mix is gonna be different for every show, so what's the point? Every night the sound is different and, if I understand the way sound interacts in air when it comes out of two point sources, the sweet spot will therefore be different.
S0, if you're so precise with measuring the exact number of feet and inches from each speaker (such as by using a laser measuring tool) then how can you possibly ensure that say three different soundguys on three different nights are gonna make sure the balance out of each stack is the same each night, the mix out of each is the same, etc?
The answer to me is that the best method is _still_ to get there early and TRY to find and setup in the sweet spot during the warm-up band and then hope that it doesn't change for the headliner. But in the end, unless you're able to move your stand around during the headlining show, finding and locating the sweetest sounding spot on any given night is totally determined by luck.