Returning a tier and rate

[GolfProRM]

I've got a section of a form where I want a user to enter a credit
score, and I need it to return a "tier" and an interest rate.

Breaking down my tier structure (using fake numbers to protect the
innocent):

Tier Score Low End Score High End
Base Rate Margin
1 800
999 5 0
2 650
799 5 2
3 500
649 5 4
4 300
499 5 6
5 100
299 5 8

When the score is entered, I want one field to populate with the tier
number, and the other to populate the result of the base rate +
margin.

The kicker is that I have two credit score fields - one for a borrower
and one for a co-borrower. If I only have a borrower, I want it to
calculate based on the borrower's score, but if the co-borrower's
score is entered, it needs to calculate based on the co-borrower's
score.

I'm trying to determine the best way to go about this. I'm not sure
whether it's better to try and query the data out of a table, or build
a function to handle it. I'm going to have to store the Base Rate
somewhere accessible as it can change often, but the score levels and
margins wouldn't have to be very available.

 

[GolfProRM]

Let me try this again

Tier | Score Low End |Score High End | Base Rate | Margin
1 800 999
5 0
2 650 799
5 2
3 500 649
5 4
4 300 499
5 6
5 100 299
5 8