Hi Gus,
I do not have the answer on which distance to apply really. Just to share my experience:
- here at home I never have had any issue with de-sensing and I do not use any filter or trick. with any of the radios (ts2000, ft817, thd7) it all works fine.
- field operations: I use no desensing filters either. 145mhz antenna straight to the ft817 435mhz antenna straight to the thd7 antennas clambed together on eachother
... well, in this set-up it makes a BIG difference how I align the antennas. Moving one forward 1 cm does make a big difference. (sliding the booms across eachother) so not the boom2boom but element to element distance. but that is applicable for arrow like setup, not for permanent mounted constructions.
Mmm, I am afraid this information doesnt help you at all :-)
Henk