Tiz Lads suggestion of X from Y would sort that issue out, but even then if you were working away in 2014/2015 you weren't attending.
Just do 85% of a number then.
My point is they'll be people that any system will be "unfair" to.
There's no easy answer and it's not a job I'd want.
My work are attempting to implement a pseudo rewards system and I've been involved in various ones in the past in a professional role.
They are inherently difficult and complex to manage and administer.
The best ones are the simple ones.
Caffe Nero. Buy a drink. Get a stamp. Ten stamps free drink.
"Well you need to have purchased £500 of items in the last three years or shop every week in the last year or have one leg that's longer than the other and be able to skip while singing ' she loves you' in Esperanto. If you can only do it in French then you get half the points when you shop on a Friday at the beginning of the month"