You can easily cut perfect circles using a circle cutting kit.
For our P51D and I-Cut100 Plasma cutters the kit comprises of a 2 wheel guide that attaches to the head of the plasma cutter which incorporates a bearing, so the head can rotate in the guide. You then have extension bars which screw into the head guide and into the centre of the hole holder which can be magnetic, secured via a drilled hole or a small indent in work. These will generally cut circles from 70mm to 800mm.
For our P30C and P31C Plasma Cutters. the circle kit supplied with 2 wheel guide to hold torch head and 1 bar and centre pivot, this will cut between 8CM and 85CM circles.