The numbers and construction for me is the easy part.

However, you cannot buy controlling interest in that LLC or even more than 30% of it (not sure about 30% but there is some guideline about it).You cannot also take any role in managing the project as it would violate IRA rules.

Is there a rule of thumb for how to go about securing deals like this?

Search 2% rule for example.

As I said, I've never done this, so is there a "rule of thumb" percentage of vacancy that I should go by?

It generally has easy to access comparables.

Website gurus don’t usually know these rules and the advice offered by some is just plain bad.

40% of the gain is my rule of thumb.

Makes it pretty easy to reconcile cash changes and all that fun stuff. 

From my research, it is true in most states unless you give the tenants full disclosure beforehand and have their consent in writing or on the lease rules and initialed.