Stay within the recommended Z height. Bits aren’t long and you won’t be able to do much thicker than 1/2" aluminum any way with bit length. You wan’t to have enough for some thicker material and clearance. 3 and 1/2" should be good. If you know that you are not going to use anything thicker than 1 inch, you could go shorter. For example, I have not done anything thicker than 25mm foam up to this point. That is even thicker than my bits so I can carve on the surface but can’t cut out.