A percentage calculation from a predetermined kick-in point

I am trying to put together a formula which is probably really simple but I am at a loss.

We have a bonus system that kicks in when a staff member reaches a given percentage. from that point they then receive a bonus based on a percentage from that point. I have worked out what I think would be correct (shown below) but I don't know how to put a formula together to do the calculation for me.

the kick in percentage is 80% and the min bonus at that would be 200.00
if their score is 100% then they would get 500.00

I worked out that each percent would be equal to 15 (500-200=300)/(100-80=20)=15

therefore

81% = 215.00
82% = 230.00
83% = 245.00
84% = 260.00
85% = 275.00
86% = 290.00
87% = 305.00
88% = 320.00
89% = 335.00
90% = 350.00
91% = 365.00
92% = 380.00
93% = 395.00
94% = 410.00
95% = 425.00
96% = 440.00
97% = 455.00
98% = 470.00
99% = 485.00
100% = 500.00

Because the amounts change according to job description and the percentage score comes from a score card, it would be great to have the following:

Cell 1 = Variable value. In this example Max 500.00
Cell 2 = variable value. In this example Min 200.00
Cell 3 = Scored value. In this example say 97%
Cell4 = Result. In this case 455.00

Is there a simple formula that I could use to achieve this result?

Thank you in advance