[TowerTalk] Two 15m yagi stacking

[BA7NQ Terry]

I am preparing make my first yagi stacking for contest station and
researching some document from internet. The tower will have 30 meters high,
and I have two 5 element yagi.

My question is:

1) The separation distance between two yagi? Some documents said is 3/4
wavelength, or 5/8 wavlength, or 0.62 wavelenght. How to consider this space
vs stacking performance?

2) My idea is, the bottom yagi fix to EU, and the top yagi is rotatable,
when the top yagi beaming to EU, then switch the two yagi to "BOTH", is this
right?

3) About the YAGI, I found many station use 4 element or 5 element yagi for
stacking,  is high-elements good for stacking or just 4 or 5 elements
better?

Thanks for reading my message.

73! de BA7NQ Terry