MTS

Ok, I'm going to take a crack at this. But it's early so....who knows?? And I could be totally wrong so this could be a totally wasted excercise. :mrgreen:

If I reread the op and the subsequent posts, I think Aully wants to transform a 3 RAIL block into a 5 RAIL block, AND have the overall size of the quilt be a bit larger. That's where all the weird numbers were coming from. See the first picture.

If I'm correct, then the rest makes sense. If not, go have coffee and ignore the rest.

I'm assuming you haven't cut anything yet (let's hope so) because I would just ignore those measurements from the pattern and start over to get something very close in size to what you want.

It's much easier to decide on how wide you want each of the strips to be, AND then cut the length based on what the total size will be when they're sewn together. That way you get a nice square block, and you're not working with 7/16 measurements. Because that is just painful. Especially in a such a simple block like the Rail Fence.

For a five rail block, if you were to make each rail 1.5 FINISHED (cut 2" ), each Rail Fence block would be 7.5" FINISHED.

In a 7 x 8 block layout with a 2" FINISHED border (cut 2.5" ), the dimensions of the quilt would be 56.5"x 64", so that's pretty close to where you started out. (2nd picture)

Now if you want to make a bigger quilt, you have two choices:
1- just add more rows to the quilt top and/or make the rows longer.
or
2- make the blocks bigger, and add the blocks you need to achieve the size you want.

You have to think about scale as well. Obviously, 5" blocks on a king size bed might not be as appropriate as 10" blocks. It depends on the pattern.

The last 2 pics are for an approx 94" square quilt top.
The first has the same 7.5" blocks in 12x12 block layout,
and the 2nd pic has 10" blocks in a 9x9 block layout.
Both create the exact same size quilt top.

Also think about the contrast between your rails.
Are they well defined so the pattern shows up clearly on a large quilt?

Picture that on your bed.

Which scale do you prefer?

The 10" blocks would have rails that are CUT 2.5" wide.
Depending on the WOF - if you can get at least 42" workable length - you'd be able to get 4 blocks from each strip set. But let's not get ahead right now.

What size bed to you have - queen or king?
How long to you like your overhang to be on the sides & bottom?
Pick the scale size you like and then we can figure out the exact number of blocks you need. Although you can figure on at least 2 yards of each of the five fabrics.
Easy. ;-)

Was that what you were trying to figure out? If yes, I hope this helped.