How much height to add to RO for a sloped sill? – GreenBuildingAdvisor

Hi all, new user here. I’m a owner/builder with an eagerness to learn. 

We’re building a workshop/studio/garage in N. Florida with Thermasteel SIPs.

I’ve been deep diving on building science youtubes and wherever I can read more in my spare time leading up to this point.

My question is about the use of a sloped sill that I’ve discovered while watching the Build Show content and others (WKD Construction) who employ this technique.

Since we’re using prefabricated panels and flangeless windows, I’m stuck on how much room in my rough opening I should spec for the addition of the sloped sill (back dam, sloped siding cut to length of sill (5/8″?) and reverse shim placed on top of that slope for the “level” platform for the european style window unit to rest upon.

If it’s, say, a 5/8″ sloped piece, then I assume flipping the material for the level platform would mean about an inch and 1/4 total? (5/8 plus 5/8?).

How common is this sloped sill practice?

Thoughts? Thanks in advance!