I am trying very hard, to shop my stash. But when it comes to a 'Special' quilt like a wedding gift, I will buy yardage and usually go by the pattern's recommendation--plus a bit. And I am really bad at liking a pattern say for a throw, but wanting to make it into a queen sized quilt.
I have one of those calculators for fabric (won it as a door prize at a quilt show) and I really have tried to use it--once or twice. But I am much better at figuring things out on paper--with diagrams where necessary.
And I have learned to improvise. I made a disappearing hourglass quilt using 1 meter of beautiful batik wide back and a very nice 100% cotton sheet from a thrift store. I had NOTHING left of the batik and just enough of the sheet for the binding. I made another quilt where I ran out of the black(ish) background but had another black that looked almost right. I ran the black fabric thru the washer (twice) with the tiniest bit of bleach to just mellow it a bit and no one has ever noticed that there are two different blacks in the quilt.