MTS

Personally, I would be more concerned about making perfectly sized QST blocks.

Dunster's math, as always, is correct - for QST blocks the starting squares are always at least 1.25" larger than the desired FINISHED size of the block.

And that works if the cutting, sewing and pressing are all PERFECT.

I always cut my starting squares bigger so I can trim the blocks, once assembled, down to the UNFINISHED perfect size.

If you find the 5.5" size is the most efficient size to cut from the fat quarter, I'd count on 4" FINISHED blocks (so the blocks would be trimmed down to 4.5" UNFINISHED).

Keep in mind, though, that scale is very important. What looks great and appropriate on a 27 block small runner placed on a 48" table will look totally different on a queen size bed.
Not necessarily wrong, but it's just something you should be aware of before you start cutting.

Here are two examples.

Assume your FQ collection is made up of perfectly cut 18"x22" FQ's (and that's a big assumption).

You'll get twelve 5.5" squares from each FQ.
But in reality, only six squares will be used in the making of the QST blocks, as the other squares will be plain - cut 4.5" UNFINSHED.

Let's make life simple, though, and assume from the 37" FQ's, you'll get the equivalent of 444 squares (37x12) - half will be QST blocks and the other half will be plain alternating blocks.

Using a 20x22 block layout of 4" FINISHED blocks will create a quilt top 80"x88".

The second pictures illustrates using blocks that are 7" FINISHED - starting blocks were cut 8.5" (with some wiggle room).

Each FQ would render four starting squares (again, half will end up in the QST blocks and half will be plain), for a total of 148 blocks from the 37 fat quarters.
And, yes, you'd have some left over strips - but they could always be incorporated into the border, or used on the back.
Using a 12x12 block layout of 7" FINISHED blocks, the quilt top would be 84"x84".

Both of these are the the same pattern, (almost) same sized quilt tops - different sized blocks.
You can test it by making some scrappy 4" blocks and tossing them on your bed to see how you like the size.

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