Subwoofer location is not too critical as the ear can't detect location of low frequency sounds. But it does matter in some cases where placement will change how much bass the thing generates. For example, corner placement with carpet will sound different than moving it a foot from the wall on wood floor, and so on.
Move it around a bit and see how things sound.
Placing something like speakers based on just some electrical measurements is like rating a dinner by counting how many peas you get and the weight of the steak.
I like your approach. Should I start mid wall on three sides and go from there? Should I move all three subs at same time?