One missing fact: are you changing the bathroom or kitchen layouts? Keeping fixtures and appliances near their existing services can make the risk quite different from moving waste lines or opening walls. Also ask whether the quote includes demolition, disposal, protection of common areas...
I’d model at least two contingency cases rather than rely on one number: 15% and 20%, then see whether the project still works at the higher figure. The contractor questions should be very specific: what happens after demolition reveals damaged substrates, inaccessible plumbing, non-compliant...