The community formula for this is a little complex. I've just run a test, and it seems to be working though. It rolls 1d20 + your dex modifier + an initiative modifier, which is different from the dex modifier (in case you have a feat or class feature that improves it. The dex modifier is defined as floor((dex-10)/2), which should give you the dex mod in the formula above, or the second value in the final formula you see when you hover over the roll result in chat. This is a cosmetic problem, and the sheet macro should really wrap that formula in double brackets so that it will display as a number in the final results. However, as far as I can see, the results are correct. If you have a 16 dex and no other initiative modifier entered, you should see a roll result that looks like: Rolling 1d20 + (floor((16-10)/2)) + 0 [Initiative Mod] = (10) + (floor((16-10)/2)) + 0 That floor formula is your dex mod and the 0 is your initiative mod (from a feat or feature). It should add up correctly if you look at the die roll and all mods.
