I agree with NativeTexan and EasyPeezy. I just make the size I want, usually it is easy to add a row or I make up the difference in my border (just depends on how many inches I need to add).
Like toverly, I want my quilt to cover my mattress/overlap the dust ruffle. I always have to add some because mattresses are thicker today. For my previous queen size bed, I typically made close to a king size quilt (allowing for shrinkage and for the quilt to have the overhang I wanted).
Occasionally I have wanted to use a pattern where I couldn't quite figure out what I needed to add. Thus far I have just moved on to a different pattern and put the original pattern on the back burner to either make another size (or figure out what I would need to add to make it the size I want it).
If the pattern is such that I need to add too many rows to keep the pattern intact (thus making the quilt to big after considering borders) - I typically add what I need in the border alone (but I still consider whether that would be proportional to my eyes). Or I might increase the size of the blocks if that is feasible (so maybe going from a 9" block to a 10" block).