My theatre space is 8.5 X 20 feet. The rear surrounds are monopole speakers seven feet from the listening position and the side surrounds are dipoles four feet from the listening position.

Any idea what I should use as a distance setting for the surrounds ?

Just split the differrence and use that as the distance setting. The whole subject of speaker delays is way overblown in importance anyway, since the effects are well below the threshold of audibility with common source material. Also, in movies, what is in the surrounds does not also exist in the front speakers, so any delay is useless in it's audibility.