Using variables instead of values - vb.net

Question

I'm having a problem with my existing code.
This code works fine,
Public Function jewel_intrate_new(ByVal duration As Integer) As String
Select Case duration
Case 0 : max2 = "0%"
Case 1 To 33 : max2 = "3.5%"
Case 34 To 63 : max2 = "9%"
Case 64 To 93 : max2 = "14.5%"
Case 94 To 123 : max2 = "20%"
Case 124 To 153 : max2 = "25.5%"
Case 154 To 183 : max2 = "31%"
Case 184 To 213 : max2 = "36.5%"
Case 214 To 243 : max2 = "42%"
Case 244 To 273 : max2 = "47.5%"
Case 274 To 303 : max2 = "53%"
Case 304 To 333 : max2 = "58.5%"
Case 334 To 363 : max2 = "64%"
Case 364 To 393 : max2 = "69.5%"
Case 394 To 423 : max2 = "75%"
Case 424 To 453 : max2 = "80.5%"
Case 454 To 483 : max2 = "86%"
Case 484 To 513 : max2 = "91.5%"
Case 514 To 543 : max2 = "97%"
Case 544 To 573 : max2 = "102.5%"
Case 574 To 603 : max2 = "108%"
Case 604 To 633 : max2 = "113.5%"
Case 634 To 663 : max2 = "119%"
Case 664 To 693 : max2 = "124.5%"
End Select
Return max2
End Function
All i want is to make my app as editable the values so i update the code as follows:
Private Sub Button1_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim x As Double = 3.5 * 100, z As Integer, duration As Integer = TextBox1.Text
Dim jj As Double = 3.5
Select Case duration
Case 0
jj = 0
Case 1 To 33
jj = 3.5
Case 34 To 9999
z = (duration / 30)
For i As Integer = 1 To z
jj += 3.5 + 2 'this will be editable in database.
Next
End Select
Label1.Text = jj & "%"
End Sub
But they don't have the same output.
Is there anyway to make my function simpler and shorter?
monthly interest is 3.5%
penalty interest is 2% after 33 days
monthly is 30 days plus 3 days grace period

Answer 1

These results should match your Select Case but you can pass in any interest rate or penalty interest. Import System.Math. Test against your Select Case.
Private Function Calculate(ByVal Duration As Integer, InterestRate As Double, PenaltyRate As Double) As Double
Dim Rate As Double
Dim Multiplier As Integer = CInt(Math.Ceiling((Duration - 3) / 30))
Rate = Multiplier * InterestRate + PenaltyRate * (Multiplier - 1)
Return Rate
End Function