Actual print area for 16x20 heat press?

I have a 16x20 George Knight heat press and I was wondering how much of that surface area is good for heat pressing shirts.

Do I need to stay at least 1" in and around the entire bottom pad.

Only because I was just seeing how far I could go to the edges today with some stock transfers and some of them were not sticking all the way onto the shirts.

So I just wanted to see if there was a certain rule of thumb that most of your guys go by for your targeted heated area.

Also if I do have a large layout that comes to the edges would you just recommend I press the shirt for more time with a higher temperature and more pressure or just press part of the layout and then move the design closer to the center and press it twice before peeling of the paper.

Or should I just try and stay in the outlined areas and not do over sized logos.

Thanks!