Here's a reformatted version of the little chart to illustrate what size round tiers are needed beneath square ones:
SQUARE.........................DIAG DIM. ......................FITS ON (ROUND):
4"................................... 5.65"................................6" tight, 7" comfy
5"....................................7.07"................................8" or 9"
6"....................................8.48"................................9" tight or 10" comfy
7"....................................9.89"...............................10" barely, 11" or 12" comfy
8"....................................11.3"...............................12" tight or 13" comfy
9"....................................12.73".............................13" barely, 14" or 15" comfy
10"..................................14.14".............................15 tight or 16" comfy
12"..................................16.97".............................18" or 20"
14"..................................19.79.............................. 22" or 24"
16"..................................22.63".............................24" barely or 26" (man that's big!)
Hope some of you find it helpful.
