My first two flips has been the contractor providing a general estimate for each area of the rehab. Based on your post, it looks like you're providing a great estimate. I can't imagine someone with experience needing any more than what you're providing since costs can vary GREATLY based on the materials and the rehab.
As a thought experiment, maybe you could add your thoughts on what you think should be done to the property. For instance, in my 2nd flip, I've allowed the contractor to come up with what he thinks is best for the property and I trusted his 24 years of experience. Maybe you could provide a couple scenarios that you think would benefit the property -- such as, adding a master suite, removing an unnecessary wall to open up a living room, adding a 1/2 bath to the first floor, etc.
I'd imagine for those of us who are relatively new (like me), it's easy for us to just look at numbers and not understand that flipping is as much of an art as it is accounting.