Choose a common finished block size
Start with the size each completed block needs to finish at in the quilt. For a small example, we'll plan three 6-inch finished blocks: a four-patch, a nine-patch and a block made from four half-square triangles. They share an outer size, but their component squares don't all use the same cut size. Keep a separate group for each block type.
List the four-patch and nine-patch squares
With ¼-inch seams, the 6-inch four-patch needs four 3½-inch cut squares. The 6-inch nine-patch needs nine 2½-inch cut squares, because each unit finishes at 2 inches. Once sewn, both complete blocks should measure 6½ inches square. That unfinished block size includes the allowance for joining the blocks into the sampler.
Give the HST starting squares their own row
Four 3-inch finished HST units can make the third 6-inch block. Using the two-at-a-time oversize method with ¼-inch seams, cut four 4-inch starting squares to make two pairs. Sew the pairs, separate them and trim the four HST units to 3½ inches. The starting squares and trimmed HSTs describe two stages of the same pieces; don't count them as two fabric purchases.
Split every group by fabric
Decide where each color goes before combining quantities. A two-color four-patch may need two squares of each fabric, while a nine-patch can split five and four. Each HST pair uses one starting square from each of its two fabrics. Add together only pieces that share the same fabric and cut size. Keep directional prints marked so they aren't turned by mistake.
Check fabric yield after the list is settled
Use the square cutting calculator for each fabric-and-size group, with that fabric's measured usable width. Separate calculations don't automatically share leftovers, so don't assume their rounded purchase amounts are the most efficient combined cutting plan. Lay out the groups on paper, record which offcuts you'll reuse, and sew a sample of each block before cutting the rest.