i guess that depends on how sure you are of your measuring abilities
also depends on where you have the masking tape and where you start cutting. too much tape on the initial plunge will gum up the cutting edge.
this is very similar to woodworking, except the side walls are bonded materials like fiberglass... so if you are comfortable with that kind of cutting, you will figure it out just fine. you just want to make sure you are not going to blow out the back side of the cut with tear out. sacrificial material to cut into will prevent that. also the outside cover will cover some of that up as well.