@rhodion sorry for the delay in my response.
Generally, if you use your "best efforts" to validate a claim then you should be alright --- reasonableness. An expense that is reasonable for the time but lacks full documentary/ contemporaneous proof, does not mean it cannot be claimed or will be disallowed. You just have to show that it is reasonable , was necessary and that you made an honest best effort to substantiate the claim -- but you do have to show how you came to that estimated cost. So I would claim the work done and show how I computed the cost --- always keep documentation as to how you did the estimation ( in case you get challenged.
Also note that unless IRS challenges a claim it doe not mean it is correct nor that it is wrong -- it is just accepted by both the taxpayer and tax collector ( and without further opinion/comment ).
Does this make sense to you ?
Hope this helps
pk
