you can cut the shape roughly and then scribe in...can be time consuming...scribe, nibble,check, scribe, nibble, check..you get the idea...does generally produce a neat job...can you not lift the pan ? this will make for an even neater job..Mark.
I never cut around and one of the things I remind the customer of, is that should they need to change the toilet pan it will almost certainly have a different shaped base. That normally makes them change their mind.