Use escrow account. You will save a lot of trouble and time lat

Unless you want to fix it yourself, then you can come up with a reasonable number for the seller.
But, if you are a new home owner, my suggestion is not to do it. Usually property damages are more complicated than they look, and it is hard to come up with a correct $amount. Trust me, I was there myself. With escrow account, you can always ask for a larger amount of money, with the understanding that the seller will get the remaining amount back after repair, they will usually say yes.