Dink,
5 degrees sounds standard, and my 10" mount from
http://www.eastshoremarine.com/dimensions.php is 5 degrees.
I was concerned more about the height and angle to see over my front top mounted remote spotlight.
I ran all sorts of calculations to determine the min height of a radar mount to "see" over my top mounted spot light at 8" away with radar beam running down at 12 degrees …. Just to prove that I graduated from Ulowell Engineering …. Figured something like I would need 9.5" mount ….. 10" it is!
The reality is that 5 degrees with a little boat roll up and down will run and detect fine for most tops. If your top has a severe rake, you might want more.
The fog density will obviously determine a safe speed.
Good luck Nick