With your updated yardage information, you just barely have enough light fabric for your plan, but you do
not have enough dark fabric for your plan.
(After my walk through, I give you two alternatives, so you can skip to the end if you don't want to read the explanation of the calculations.)
Here's a walk through of how I calculated this.
1. How many blocks are needed? "I want to use 5 (12" blocks) across the top & 6 (12" blocks down the side)." This means 5 x 6 blocks, which is 30.
2. How many of each fabric per block? "I want to make each square with 5 dark fabrics & 4 light fabrics." So for light it is 4, and for dark it is 5.
3. How many total squares of each fabric?. Multiply the number of blocks (#1) with the number of squares per block (#2).
So for light it is 30 x 4, which is 120, and for dark it is 30 x 5, which is 150.
Now we start to figure out the length of each fabric you need.
4. Measure the
usable WOF - I will assume that your selvedges are 1" wide on each side. So the light WOF is 42 and the dark WOF is 40.
5. How many squares can be cut from a WOF strip? Take the WOF measurement (#4) and divide by the square size (4.75) and then round the number
down to the nearest whole number. So for light it is 42 / 4.75 = 8.84, which rounds down to 8, and for dark it is 40 / 4.75 = 8.42, which rounds down to 8.
6. How many strips of each fabric do you need? Take the total squares of each fabric (#3) and divide it by the number of squares in a WOF strip (#5), and then round the number
up to the nearest whole number. So for light it is 120 / 8 = 15 (no rounding needed), and for dark it is 150 / 8 = 18.75, which rounds up to 19.
7. What length of fabric do you need? Take the number of strips of each fabric (#6) and multiply by the square size (4.75). So for light it is 15 * 4.75", which is 71.25", which is
just barely less than the 2 yards (72") that you have.
For dark it is 19 * 4.75", which is 90.25", which is more than the 2 1/3 yards (84") that you have. So you do not have enough dark fabric for your plan.
Alternatives:
A. Do you want to make a quilt with fewer blocks instead?
You have enough dark fabric to cut fabric to cut 17 WOF strips, which would give you 136 squares, which would allow you to make 27 blocks. So you could make a quilt with 6 blocks by 4 blocks.
B. Do you want to make a quilt with smaller blocks instead?
If you switch to cutting all your squares 4.25" inches, you can get 9 squares per WOF strip, so you need fewer strips (17 for dark) plus each strip is narrower, and so you need only 80.75" of the dark fabric. Cutting your squares 4.25" will give you a finished block size of 10.75". (Or you can cut all your squares 4" inches, and get a finished block size of 10". I got the information on calculating block size
here.)