More on How to Figure the Right Lengths of Moulding to Make a Picture Frame
Why are you Adding the Width of the Moulding times 8 to the Frame Size?
When building a picture frame, you will miter the ends of each section to create the angles that when fitted together will form a corner. Each one of the eight miters needed to make a four corner frame will be equivalent in the length to the width of the frame when measured across its face. So if the frame is 1" wide, you will need enough moulding to make the four sides as well as 1" x 8 to account for the miters. If the frame is 2" wide, you will need 2" x 8 = 16" to account for the miters.
Why are you Adding 1/4" to the Length to Account for "Wiggle Room"?
If you want to frame something that is 16"x20", you could make the frame exactly 16"x20". But then each of the frame's components would have to be cut perfectly to fit into the frame. If they are off by just a hair, they won't fit. In picture framing parlance the wiggle room is called the allowance, and every picture framer builds an allowance into his or her frame. Typically the allowance is an additional 1/16" along each side. To have enough moulding to build an addiitonal 1/16" allowance into the frame, you are going to need (4) x 1/16" = 1/4".
Why will (2) 4 Foot Lengths be Appropriate to Make a Picture Frame 16"x20" that is 2" Wide?
Consider that you will need two sticks of 20" (16+2+2) , and two sticks of 24" (20"+2"+2"). Now, if you attempt to take both 24" sides out of a single 48" (4 ft) length, you will be cutting it too close and not have enough for the allowance. The better approach is to take one of the 24" sides and one of the 20" sides out of each 48" length. This is a total of 44-1/8" per 48" length, leaving plenty of stick to work with.