If you’re using a chopsaw to cut your trim, you’re already ahead of the game. Here’s a technique to make everything fit even more snugly. Cut your trim section long to begin with. Then, after checking the fit, shave off 1/32″ increments. Here’s how. With the saw off, lower the blade and slide the stock up to the idle blade. Then, holding the stock in place, raise the blade and make the cut. Since the blade teeth are 1/32″ wider than the disk, you’ll make a very slight shave.
It’s a good idea to start long whenever possible. If you’re coping or mitering a joint in baseboard or ceiling trim, leave the piece long until you’ve fit the joint perfectly. This way, if you make a mistake, you don’t end up having to cut the piece shorter than required and wasting trim.
Since trim work is fine work, your measurements should be as accurate as possible. If you can’t fit the stock in place and mark it for cutting, use a folding wooden rule. For up to 6′, this is a better tool than a tape measure because it’s rigid; it won’t flex and throw off your measurements. Get a good wooden rule with a brass extension, and you don’t even need to measure. Here’s how. Unfold the rule until it’s just shy of the distance to be measured. Slide out the extension and hold it in place. Place the rule against your stock and mark the cut.