The dark arts of carpentry (and a belt sander) can make a too-short board, not too-short anymore (but not longer)
Everyone knows about using a scrap of baseboard with a miter cut on the end to use for measuring to the long point of the opposing piece. Most people even know to put opposing miters on either end of the scrap so it can be used for both corners.
Some people even account for how out of square the corner is in their bevel cuts.
But what happens when you cut the last piece of baseboard in existence TOO SHORT?
That's when we must to turn to the dark arts of carpentry magic to fix the problem.
But seriously, folks. How can you make a too-short piece of baseboard magically longer?
By reducing the thickness of the other piece, effectively moving the short point of the angle to bring the long point in closer to the drywall.
Mark the opposing piece at the top of the board about the distance that the other piece is short, 1/16-inch in this example. Taper that mark from 1/16 to nothing over about 12 inches.
A good way to do this is with a belt sander. to keep the sanding consistent, make a similar guideline along the bottom of the board, too.
—The Funny Carpenter makes videos about Living Space Improvement and Tools and he posts them on his youtube channel