Take number of diamonds across for size quilt you want...__________

Times number of diamonds down for the size quilt you want...________

Then divide that total number from above  by the number of diamonds you are getting  from each strip ( generally 7-8)

In other words, say you want a quilt 78" wide by 88" long.

Multiply 20 x 26 (the number of diamonds down and across).  That is 520 diamonds.
If you get 7  cuts per strip set, divide 50 X 7 = 74.28.  So, you will need 75 (round up) selvage to selvage strips for that quilt.

I have the numbers in increments but if you want a size in between, each diamond is 4.24 long and 3" wide with the seams taken up (sewn).  3.5 inches when you cut, remember?

You need an even number of diamonds across.

You can have any number down, unless  you are adding the "built in" border.  Then  you need to have an ODD number of diamonds down. Just take apart some of the strip sets and sew one diamond to the bottom of each row before sewing on the built-in border sections.

Remember also, that you may cut off the top and bottom "peaks" when you put on the binding and that will take off a little of the length.  So, sizes are approximate due to the whacking off of the top peaks.  Unless you want to bind the peaks too!

Remember, border either plain or "built-in" will make your quilt larger too.