I recently created a 4.5 x 4.5" card and wanted to create an envelope in which to mail the card. Since the post office charges a surcharge (I believe $.20) to mail a square envelope, I wanted to make a true 5 x 7 OE. This will not hold a 5 x 7 card, but it will hold my 4.5" square card.
Using the Martha Stewart Score Board, I took stock that is 9.375 x 9.375 square. Scored at 3.125 and 4.125 and viola... a 5 x 7 OE.
Just thought I shared, since I did some calculations and trial and error to figure this out.