As I explain in my lectures for FM (and for Paper MA, because this is revision of MA), you can use any two ‘guesses’ – it does not matter. I also explain that different guesses will give slightly different answers (because the relationship is not linear and any answer is only approximate) but that this is irrelevant for the exam.
I suggest that you watch my free lectures – they are a complete free course for Paper FM and cover everything needed to be able to pass the exam well.