You have to make your best estimated guess on what development costs are going to be and do you best to stick to those numbers.

My company has been doing residential Spec builds in Austin for the past 6 years or so and recently got into doing Custom builds, an architect we have worked with in the past approached us and basically he wants us to build a house he's already designed for his clients, (we are still in the process of meeting with them and doing estimates etc.), we verbally agreed on a 20% GC fee over construction costs, but I have a feeling he will want to split this 20% fee with us somehow as he tried to do something similar a few years ago in a project that didnt end up happening (he wanted to split the 20% in half between us) which we didnt think that agreement was fair at the time.

I'll write down the rates below. 2.75% - 0 lender credits - $1,327/mo mortgage2.875% - $1,200 lender credits - $1,340/mo mortgage3% - $2,023 lender credits - $1,337/mo mortgage3.25% - $4898 lender credits - $1398/mo mortgageI'm leaning more towards the 3.25% so I pay close to nothing for closing costs but obviously with the estimated rent, I would have to charge $1,600 to cash flow maybe $100.

We estimate that it would appraise for about $450K now!

I have the funding available, but the only rehab estimation I have knowledge of is through the books/podcasts I have listened to. 

Listing site estimates roughly $1,700 in mortgage, insurance, & taxes with 20% down.What are your initial thoughts?

I for one will not even attempt to perform a estimated cost of repair or a ARV.

(The reno costs are a very rough estimate... just trying to run scenarios and experiment with the process).

I'm estimating it would cost $60,000 and 6 months to complete the job. 

Hopefully they will provide a good estimate on you permit fee schedule.