Hi BuckEye Rose
I had a very similar situation with a quilt for my SIL a few years back. Prewashed fabric and batting, carefully measured and endlessly remeasured
during it's construction. The end result was 3 or 4 inches too narrow (probably too short also, but that wasn't an issue). The fix was easy - I simply sewed a 4 inch strip of fabric around the dust ruffle, so the black binding lays against a black background instead of white muslin (eek!). It seems obvious now - but at the time I was totally irked .
The original idea was for the binding to barely touch the beautiful cherry wood side rails and peaking out below that is a pretty cut work dust ruffle. The look was totally lost with the black binding falling 2 inches short of the side rails.
Fast forward to small quilt made earlier this year. I was idly measuring the quilted and unquilted 9 patches - the quilted sections were about 3/8 smaller than the unquilted sections. Along with careful laundering that is certainly enough to account for a loss of at least 3%.
Note to self - never commit to an exact measurement without some built in wiggle room.
Good luck with your quilt.
Lefty - in Philly