Originally Posted by szyquilt

I appreciate all your advice.

I did go to the shop with my measurements ready and only changed my plan based on the shop owner's advice.
I hadn't planned to buy the backing during this particular trip, only the borders. I should have double-checked her measurements before buying the backing.

I'll just count this as a learning experience. I'll use what I have, make adjustments to my borders to make it work and stick to "my" plan on future projects. Thanks for listening to my vent!