I want to make a frame for an 8×10 picture. I think I need a 3/8 by 3/8 dado i would like to use 1×2 stock (3/4×1 1/2) half lap at each corner. How do I compute how long each side? And should I make the dado before cut to length?
I may be mistaken, but I believe you take one side at a time, using the inner corner of the miter lined up with the edge of your frame. If you cut your frame sides to size and then try to put it together, you leave yourself little margin for error and less room to “micro-adjust” as you put the sides together.
I am about to take on the picture frame series in earnest but would point out/recommend watching him do the edge of the chessboard project. He starts with 1 long strip of mahogany and cuts it one side at a time, ensuring that each miter fits perfectly against the one next to it before moving on to the next side. IT is especially critical when fitting in the last corner.
That all said, you said you are joining with half-lap joints so perhaps the fitting is a bit simpler but you still want to make sure to measure against the inside corner of the frame, NOT the outside–you can always plane down the outside when you are done to make everything flush. If the inside is out of alignment you can’t adjust and your frame will just look….not as good.