Sometimes rebuy tournaments go very well at the beginning and I accumulate a lot of chips, when I've already got lots of chips it seems silly to pay more money for the add on - usually costing the same as a whole rebuy - to top up my stack by say, less than 20%.
How can I assess whether an add on is worth taking or not?
Is there a generalised threshold something like, if my chip stack is more than 5x the add on, there is no point in purchasing it?