change field value for car loan calculator

Hello,

I've made a couple of loans, and mortgage calculators (the FP templates ar not adapted, in europe, Belgium loans are calculated with other elements than what you find in the templates that FP has to offer.)

When i have calculated the loan amount, i show the annual cost %, (these are fix %, and visitor can't change them)

The % depends

1. the loan amount, between 2500€ and 3600€ it is 12%, between 3601 and 5000 it is 5,25% and so one ( 5 different %)
2. and the age of a car

now i have 5 fields with filled in with a % and blocked so people can't change them, and then i use the conditional logic so the appropiated % is shown.

But i want to do this with less steps, i've read somewhere that i can work with If field 200 is= amount than field 201 value is..... how could i achieve this in my case

example

Field 1: car age = New

Loan amount = 1300€

The ACP  % is 3,25%

another example

Car is older than 5 years - Loan amount is 6500€ so the ACP is 9,99%

the hight of the loamount is 2500<5000 / 5000<X<7500 / 7501<x<10001 ....

how can i achieve that i use one field for Annual Cost % and that depending on the 2 elements the correct % is filled in automaticaly in this field?

I hope i made myself clear?, any tips ideas ar more than welcom