If I were buying for future quilts and the quilt might be a queen size and I judge there would be 3 fabrics in the quilt (think rail fence) then I would do the following estimate.
Queen size is 90x108" or there about. This is two widths and 3 yds long. Or 6 yds of fabric. Say the pattern is simple I would add 2-3 yds for seam allowances which makes 9 yds.
9 yds using prints is 3 yds each.
For lap size I usually buy 1 1/2 yds of 3-4 prints for the tops and then additional for backing.
If you don't have a specific design in mind you just have to "gut feeling" the amount you will need. You say you are going to lancaster Pa. Do you visit Sauders which is north in Denver.
Pa. Have you been there. If not, you should go. It is not far north of Lancaster.