[TowerTalk] Yagi spacing

[Barry Fox]

I am planning to put three yagis on a single mast.
Closest to the ground will be a 2 elem 40M, then a
13-55 MHz LP in the middle and a 2M beam at the top.

I just took a look at the ARRL web site where they
recommend minimum spacing for this sort of thing,
and it looks like 1/2 wave at 20M should be the 
spacing between the 2 ele 40M and the LP.  That's
kinda like 34 feet, a bit large, I think.

What is the collective list wisdom for spacing 
these three antennas?

TNX to all,
Barry - W1HFN