So I'm loving the tube strip method, but have not figured out how I can end up with a specific size block.

If I'm using 2 jelly roll strips (2 1/2" ) my finished squares are 6".

I want to make finished blocks 3 1/2" blocks so that I can make 12" blocks (12 1/2 unfinished).

How do I figure out what size to cut the two strips? There must be a formula ...

Yes, my terrible math skills have caught up with me :(
Thanks!

Your 4 blocks need to be 4 1/2 inches unfinished

If you are using two strips they need to be cut at 2 1/2

If you are using 3 strips then two are at 1 3/8 and the third is 2 1/2

:D:D:D

Thanks Amma - but the blocks I made with 2 strips that are each 2 1/2", sewn to one 4 1/2 strip (to make one tube), cuts into squares that are six inches.

So, your 12 1/2 inch block will be made up of 3 1/2 inch blocks? In other words, 16 tube blocks (4 across x 4 down) to make up your 12 1/2 inch block? Or am I confused?

I have never made this, but now I'm confused.

Be careful as these blocks are cut on the bias and the larger the block the harder to handle they will be.

Yes, exactly ... 4 blocks (3 1/2 inch) by 4 blocks!

Lots of starch and careful handling to control the stretching.

I'm thinking your strips need to be 1 1/4" but haven't had time to actually try it. You said you want them to be 3 1/2" finished? Or is that the size before you sew the blocks all together to make the 4 x 4 - 12" square (12 1/2" unfinished)?

You're right - 3 1/2 " x 3 1/2" unfinished, put together in 4 x 4 to yield 12 1/2" unfinished block.

I did enlist my trusty geometry expert (also known as my favorite husband) who did lots of squares and roots and hypontenuse calculations to come up with needing strips 2.45" plus the 1/4" seam. So my 2.75 strips, (or 2 x 1.5" strips) get me close enough to allow me some squaring up room to end up with those 3 1/2" blocks. Whew!

Ah ha - I forgot to add one of the 1/4" seam allowances. Be sure to post pictures please.