Help please from the quilt math wizards here
 

I am designing a quilt to make next and I just can't seem to get the math right.
I made 4 Patch Posie blocks that squared up at 5". I want to set these as a square in a square, and I can get that ok. I believe these will square up at 7".

I want to alternate these with a rail fence block with 4 rails. I can't for the life of me figure out what width to cut the rails so these blocks will square up at 7", too. I'm blaming the fibro fog... lol

Thanks in advance for your help!!

If the finished block is 7 inches, cut your logs at 2.25. If the unfinished block is 7 in you will cut logs at 2.125
thats 2 1/8 inches wide. Hope that works

Oh thank you cjsews!! It helps immensely. I am eager to get started on this one and spent the whole day trying to figure it out. I cut 3 different sizes and none of them were right. It's the unfinished block that is 7". Hugs and smiles!!

When I am making 2 different blocks that I want to match I get out my graph paper and draw up the finished block and then just add 1/4" all around the pieces. Good luck and please share a picture on your progress.

Yes, I had it all drawn up on graph paper, but because of the kind of odd sizes involved, I just couldn't get that width of the rails to come out right. cjsews gave me the exact right answer and my blocks fit together exactly!

For progress so far, I have all the rails cut and am sewing them together. I also cut the squares that will go around the 4 patch posie blocks. You can see in the photo how it's sort of going together; the rail fences and the square in a squares. [ATTACH=CONFIG]606879[/ATTACH]