Yeah, a shim is often required, in more than one way... the transom probably isn't perfectly flat! So lay the block up and bring a something square along too. You want it flush, and wide enough that the bolt holes won't crack the grain if its wood.
(I've got an aluminum
block that I've knicknamed "The Space Shuttles Wheel
Chock." My garlick mount did not have trim... this summer I will be replacing the whole unit.)
Also, make sure the boat is loaded and people "weight" is in the cockpit
when you decide the depth
to mount it. This means hanging the outboard
and mount via a rope
, and checking the flange depth
. Go deeper than you think is needed, as any hobby horsing will begin to cavitate the prop.
Good luck, there are a lot of variables to think about... but if your slip is big enough to handle it, set up the height so you can drop the depth down a few clicks and tilt it out of the water
as needed. (Let you motor through a little bit steeper waves than having the lowest setting be the "proper height.")
The easiest way to measure the angle, is to use a slope formula by dropping a plumb with a pre-marked string with one inch intervals off the top of the transom. Just knowing the angle doesn't really do it when you go to lay out the spacer... it'll show the how far off flat the transom is. That is... if you you want to compensate for some angle in the spacer too.