Well, let's use your batch and your target carbonation level in the calculator, and see where it takes us.
First, Boddington's is a classic English Pale, so we see from the pulldown styles menu that carb levels on the order of 1.5 to 2.3 atmospheres are typical for the style. Let's pick something right in the middle, like 2.0. Enter 2.0 in the Desired Volumes spot.
Next I'll assume your braggot is at a reasonable room temperature of, say, 72F. Enter 72 in the temperature, and leave the F degrees selected as-is.
Next I'll assume you're working with a US gallon, so enter 1 and leave the pulldown on gal (there are also selections for Imperial volumes, if you happen to be English and "old school.")
You said that you want to use corn sugar, so we'll leave that default selection set as-is.
Now unless you have vigorously de-gassed your must, it probably has somewhere between 0.5 and 1.0 volumes of CO2 already in it, left over from fermentation. Again, we'll pick a middle number, so enter 0.75 in the volumes spot. NOTE: Although the two examples given in the table (0.9 for an ale at 65F and 1.2 for a lager at 50F) are both higher than my assumption, I've found that the act of racking braggots (and other meads) tends to release more of that residual than often happens with beers. I'm not sure why, but I've always had good luck with the 0.75 volume initial assumption, so that's what I routinely use.
Then hit calculate, and the answer is: 0.639 ounces (0.64 is close enough), or 18.1 grams (again, 18 would be close enough).
You ended up with 0.9 ounces, which would be correct if you'd used the default American Amber style and assumed zero atmospheres of residual CO2. In reality, any amount between the two values, 0.64 to 0.90, would give you an acceptable carbonation level and would not be in any danger of creating bottle bombs. Priming doesn't need to be an exact science - at least not as exact as some of the other things we do in meadmaking.