Also, make sure to connect with local contractors to get estimates on your prospect properties.

Knowledge and use of this information should help you to make a more realistic cost estimates for your models. 

Now this property is in a class C neighborhood with a calculated 11% cap rate estimated.

Estimated CostsADU costs can range DRAMATICALLY depending on the state of the lot, custom vs fast track plans, size of ADU, materials and finishes, speed and quality of the builder, etc.

The contract must contain:the contractor’s name, address and telephone numberthe approximate start and completion date, including any contingencies that would change the completion datea description of the work to be performed and the materials to be provided to the consumer, including make; model number or any other identifying information; and the agreed-upon price for the work and materialsa notice advising you that you have three business days to cancel the contract, unless during an emergency you waive the right in writinga notice that all payments received by the contractor prior to completion of the contract work must be deposited in an escrow account

I’m talking about getting thorough with expense estimates, factoring in property taxes, insurance, and a cushion for maintenance.

As @Troy Gandee mentioned use the tax estimator on the County website or call the assessor.

The pro-forma estimates just don't  provide enough information to plan long term.

I've read many of the BiggerPockets books like - House Hacking- Flipping- Estimating Rehabs- Multifamily Millionaire Book I and II- How to Invest in Real Estate with Low or No Money Down- Investing from a Distanceand even some outside of themand have been wanting to do something in Real Estate for awhile.

They should be able to determine the COC, annualized return, estimated ARV of a BRRRR, accurate rent comps and more.