For regular sized FG I use Eleanor Burns' flying geese rulers. I have two, one for 4x8"/2x4", the other is 3x6/1 1/2x3. For other sizes I use the Lazy Girl No Math or Marti Michell's Perfect Patchwork. Each has its advantage, but the Eleanor Burns' has the advantage of trimming to the right size as the last step. I taught a workshop on FG units at our quilt show -- five ways to make Flying Geese units. Eleanor Burns' Quilt In a Day Flying Geese Rulers are the best, but most costly. You can also get them in other sizes. Lazy Girls' and Marti Michell's are one ruler for many sizes.