Since NAR always has an up bias, I'd say 8% = flat to slightly down.

For capex its better a general flat fee number based on home size or if its a condo or not. 

Also as far as management, I have a couple of property managers in my pocket that will do it for $50 a door instead of a flat  %.

However  the cabinet boxes come put together at menards and ikea are flat packed (extra work).  

With that said, I have seen some investors approach the deal as a wholesaler would, find a below-market deal or motivated seller to get the property under contract at a discount, and then just get a handyman in to get the property into "liveable condition", touch up paint, make sure everything is functional, do an intense cleaning, and then re-list the property for a small profit after closing costs etc, maybe using a flat rate listing service. 

That should be a flat bonus, that way they are not encouraged to hurry their efforts. 

A "flat" section of the roof was completely redone.

You will never stop making "mistakes" until you quit RE investing altogether.

The agent could be transaction broker and do a flat fee of $1,500/$2,000 to get it done.

Pre-constructon contracts are risky if you don't have experience and can be problematic if you have cost overruns or flat out miss the numbers.