Usually with meads, I'll add a little more honey that I think I'll need, then dilute down to the gravity I want. This is because of the issue noted above that honey varies GREATLY even season to season, brand to brand, location to location.
For something more complex like a Braggot, a better idea is to "proof" that batch of honey by making a very small sample to test with a hydrometer. Like gradually adding honey to 10oz of water, an ounce at a time. Then note the amount when you hit your spot. This way your not diluting your malt as with the method above.
For a braggot, I might expect to proof around 1oz honey to 8oz water (while like 3-5oz for heavier meads).