sanderdvd, to find the optimal setting you would need to do multiple measurements of sub, L, C and R at different locations. Then vary the sub delay (move the impuls response in time) in your measuring software and sum (important: complex sum not just magnitude response summation) sub + L, sub + C, sub + R. Do this for every sub delay setting you're interested in.
Then look at your 90+ graphs and pick the delay setting that gives the best overall results
My speaker positions cannot be changed much (only max 30 cm to the left right and back and forward per main speaker). My subwoofer position is also fixed and not possible to change. My seatings are at a fixed position because I own a special custom build D-Box seating (which is awesome btw!).
What do you think about the 4 conclusions?