Hi everyone

I'm a newbie on here so I hope I am posting in the correct section.

I have (for me) quite a complicated problem to solve. I work for a company that gives away Reward Tokens when their customers buy a certain amount of product.

The "base rate" for a Reward Token is 65 (yes, it's a UK company). Therefore they
might have a product for say 10 with a Reward Token value of 100%. If they spend 65 or over on these products at that price, then the customer will gain 1 Reward Token and any remainder will be retained towards any further Reward Tokens.
Now for the same product, they also offer another price of say 6 but with a 50% Reward Token value. Therefore the customer must spend 130 or more before they will gain a Reward Token.

So far, so good, but what I need to be able to calculate is, if we want to offer multiples of prices with varying Reward Token values, what formula would I need to use to do this.

For instance:

1 widget 10 100% Reward Token Value (Must spend 65 to get a RT)
1 widget ? 90% Reward Token Value (Must spend ? to get a RT)
1 widget ? 70% Reward Token Value (Must spend ? to get a RT)
1 widget 6 50% Reward Token Value (Must spend 130 to get a RT)
1 widget ? 25% Reward Token Value (Must spend ? to get a RT)

I hope I have been clear with the description. Any help would be gratefully received.