I'm new here and i'm looking for guidance.

I'm honest.. i'm not good at Math ( i wish ! ) and i need help trying to figure out a formula for a simple calculation ( i hope is simple.. )

The case is this:

I have an initial increasing constant number which is 85 and the final number which is 95.

The produc result of the formula should be.. 0.5 for 85 and 0.3 for 95.

as the 85 increases the result decreases Being as:

85 ( unknown formula ) = 0.5

85.1 ( unknown formula ) = 0.49

85.2 ( unknown formula ) = 0.48

85.3 ( unknown formula ) = 0.47

..

..

..

..

95 ( unknown formula ) = 0.3

I've been trying so hard guys, seriously but i can't figure out.

If you can throw any light i'll really appreciate it !!

Best Regards !!