my experience with mead is less head space the better
but I have only done 3, so take that with a grain of salt
One thing I learned with wines, is you need to have some smaller bottles around. Some wines need to be racked a few times and you will lose volume. how to make that up is to have some bottles around you can rack into that are not full gallon size. that way you can compact as you compact
I have started out with 5 gallon batches and ended up with as little as 4 and a fifth. so instead of diluting the wine I started to use some odd size bottles and work down till the wine was ready to bottle
hope this helps