How much mash for a 5 gallon still

[CornDaddy231]

I recently purchased a vevor 5 gallon still to start learning the process of distillation but I'm unsure exactly how much mash I should run through the still. Iv read that you should have about 20 percent headspace during the fermentation process to keep the lid from blowing off due to pressure. I also am not sure how full you should have your still when you start distilling. I bought a grain kit for 5 gallons of mash but im debating on only using 4 gallons of water to keep some head space. Any tips or ideas would be greatly appreciated!

Thanks yall
Corndaddy231