Since the applicable laws are state specific, I recommend you invest in getting the legal opinion and advice of a well informed attorney with an in-depth knowledge of the laws, rules and regulations promulgated by the state real estate commission before commencing your wholesaling business model.

Back when my main gig was construction, I did a lot of Tenant Improvement (TI) work in structures like these. 

So using that as a rule of thumb, if you have about $150-$200k invest you can definitely see your goals met depending on the market.

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?

While there are truly amazing resources available, none of them seem to be really equipped to handle the nuances behind the content without the help and involvement of the community or retailer themselves.This is especially true when dealing with the laws and the rules promulgated by the various state and federal agencies.

40% of the gain is my rule of thumb.

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.