I have  purchased them at 40% of list price and the deal was marginal and I have also bid over asking and the property was a great deal

It is no one's fault but the experienced rehabber realizes this is likely and has addition funds set aside (and additional margin) for the associated additional effort.In summary there is no argument for thinking the realtor's rehab estimate is likely to be more accurate than the contractor's estimate. 

This isn't a huge margin, and if something comes up then it could cost me to flip.

The occupancy rates will increase some as some people book last minute but using occupancy at near start of month has some safety margin built in.What this snapshot does not make clear is whether there is significant variation by month.

Margins are kind of tight here for a true BRRRR deal.

With wholesaling, you're looking at the after repair value and the ROI for the investor buyer.In the simplest of examples, say that $100k property would be worth $200k after $40k in repairs, you put it under contract for $80k (the seller gets $10k profit) and assign the contract for $15k, that leaves a good profit margin for the investor buyer.

Your fonts are too large and the sides margins are too small

Maybe 165 would be the ARV for a normally laid out house on that lot.You've probably already considered this, but I thought I'd mention it.If it were me, I would have to have a pretty good margin to jump into something like this.

The outcome would be a sizable profit margin to split up between myself and the builder.

No, it will negative cash flow, big time.A HML will not loan on a property that either: is not rent-ready; is not priced low enough to compensate for repairs with a profit margin built in; does not cash flow with a margin for selling costs built in in case they have to foreclose on you; or that will not yield enough rent to provide that cushion.