There is a lot of room for personal preference with seat sizing, but as a general rule it is better to error on the side of ordering a seat size that is too large rather than erring on the side of a seat that will be too small.
Things to Consider
Tucker adds 1/2inch to their seat sizes (due to thick seat padding)
16.5 Tucker = 16 normally
Barrel riders usually take 1 less on the seat size
16 Trail Rider = 15 Barrel Rider
Ranch (slick seats) usually ride smaller (1/2inch seat size) so they dont slide around
16 is for most men, 15 for most women
Trail Saddles with High Cantles (5) make the seat sizes feel smaller
Trail Saddle with 5 cantle will feel smaller in the seat than one with a 3.5 cantle
English Saddles dont convert exactly but often are 2 bigger
18 English = 16 Western
Riders with inseams under 30 or 5 tall and under often need custom cut fenders
Tall riders with long legs but lighter weight may need custom cut fenders as well
Checking the Fit
General Guideline is about 3 fingers between your thigh and the swell.