Based on your post and other comments, it sounds like your rehab is extensive enough that the property isn't habitable.

Since you know what you need to buy it at and an average rehab cost, I would say $50-70k for a cosmetic flip, set the ARV based on the sample you choose and doing comps and ask them if they would invest in a deal similar to this. 

It has been best to anticipate wildly different ARVs based on not only the property details, but also the quality of the finishes that are used.

They should be bale to guide you more based on what you are looking for in the area. 

I've been a licensed agent here in Maine at the Augusta-based Pouliot Real Estate for the last 2 years, but began my real estate career in Savannah, Georgia (my GA license is currently inactive).

I'm California-based where there aren't any real deals in my area, so I'm exploring Detroit as an alternative real estate investment market - due to the combination of it being a strong market and my having connections in the area. 

I'd underwrite the deal as any other - make sure you are conformable with the quality & tenant base. 

We only have one small property, so I manually adjust the price based on what I see for the month. 

We've invested around $50,000 in improvements (central air and hardscaping) and our mortgage is a 30-year fixed at 2.5%, with a PITI of $2400.We think it could rent for $3,750 - $4,000 per month based on local comps.

Allocate equity based on contributions: you provide the $235K HELOC and management, while your former boss contributes renovation expertise and labor.