What you want to focus on, and be great at, is knowing the after repair value, and being able to accurately estimating renovation costs.

Does anyone know or have some good advice for accurately calculating the repair breakdown?

-Make sure you know all the questions you need to ask the potential seller to determine why they want to sell and to see what are any issues they have and how you can solve them-You'll need someone to help you with rehab estimates if you don't know how to do it and you'll need someone who has access to the MLS so that you can figure out an accurate ARV. 

I highly recommend getting that done in the form of a subcontract agreement - short form or master depending on the dollar value of the contract.You need to incorporate a schedule of values breaking down the contract sum into tasks, and with an accurate cost assigned to each task.

I am trying to determine if $120/sq ft is accurate for me.

How accurate should the cap ex number be?

You need to accurately estimate the amount of repairs you will need to perform.

I also prefer rentometer to rent zestimates, they're not usually too far off but I've found rentometer a little more accurate.   

To get an accurate picture, you need several of them to determine actual performance of the property.  

I feel this is fairly accurate and I am not likely to have any surprises (keeping my fingers crossed!).