When measuring, the old adage "measure twice and cut once" can be excellent advice. Pros, however, have their own variation; they usually only measure once, but then will read the measurement twice. They will mark the cut line, then cut the wood just so the saw kerf is touching the line.
Keep in mind that your mortgage interest and real estate taxes will be deductible. A qualified real estate professional can give you more details on other tax benefits and liabilities.