Hi tzaluski and welcome to the forum,
Could you provide more information on what exactly you need help with?
If its calculating the Discount percentage then something like this may help:
min ( 50 + ( Years * 5 ) ; 75 )
This will add the number of years multiplied by 5, to your 50%, but will stop incrementing when it reaches 75%.
If there is more, or it is something completely different then let me know.
Thank You! Working great.