Curing/Jar wuestion??

Now when I put into the jar, a I filling the jar up to where the bud is packed or am i leaving some room so the bud can breathe?....I tried a grow last year and my bud was all spongy like and was horrible and smelled like hay, so I'm trying to get this right!

Ideally you want to fill the jar like 80% full. I have a bunch of different sized jars but if I'm short I will leave a big stem on the nug. Problem is, you have to get a feel for curing before you can just toss a stem in there to make up for lost room. 70-80% for sure.