Measurements are important to any job, and particularly in finish work. Use your level, your rules, your squares and your tape measure as necessary. But there’s another instrument you should bring into play as much as possible. Your eye.
In an older home, or even a newer one, where settling has thrown walls a little out of whack, you might find that plumb door casing looks lousy, because the door opening itself is far from plumb. The trick here is to rely on what looks good. Follow the opening, leaving a consistent reveal, and trim the casing to fit flush to the floor.
Likewise, if the floor slopes from long-ago settling, you can end up with quite apparent gaps. But remember, you’ll notice a tapered gap below a door before you notice that the door isn’t cut square. The trick: trim the bottom of the door so it runs parallel to the floor, even if this is far from a right angle.
Maybe you have a deck that extends from the house, but it’s not square to the house. Rather than running boards perfectly perpendicular to the deck framing and having a gap at the house, run the boards parallel to the house. Then trim to the framing.
Even a well-coped and fitted baseboard may open up over time. If this gap is directly in front of you, you’ll notice it. But if you’re looking along the coped piece toward the gap, you won’t notice it. This is why trim carpenters often install the uncoped baseboard opposite the room’s entryway.
Of course, there are exceptions to the “rule of eye”. If you hang a door or cabinets on an angle, you’re going to have problems. So keep these plumb and level.