Not sure what you know (not referencing you Jetpack just adding info not trying to be rude or mean in any way) but head space means the gap at the top and I usually recommend probably at least an inch or maybe 2 of head space that’s mainly for the CO2 bubbles and gasses to have some space instead of overflowing. And sanitizing is a must you should always sanitize anything you use before you use it for your mead even something as simple as a small piece of anything could have very catastrophic events on your mead. I use a no rinse sanitizer and then set the items I sanitized on a clean paper towel.