VBA combobox options based on another combobox - excel

Question

I have a combobox with options from 300 to 650 with increment of 10 and I need another combobox to display options -5 of previous value and -15 of previous value so in case the first combobox selection is 300 then the other combobox displays options 295 and 285. I know it's possible to manually write down all the options for all the cases but it would be insane. I wonder if someone has a simple solution for this?
Private Sub UserForm_Initialize()
ComboBox1.AddItem "300"
ComboBox1.AddItem "310"
ComboBox1.AddItem "320"
End Sub
Private Sub ComboBox1_Change()
Application.EnableEvents = False
ComboBox2.Clear
Application.EnableEvents = True
Select Case ComboBox1.Value
Case "300"
ComboBox2.AddItem "295"
ComboBox2.AddItem "285"
End Select
End Sub

Answer 1

Fill Combo Boxes
For fast loading data to a combo or a list box, it is recommended to use the List property with an array of values.
The Code
Option Explicit
Private Sub UserForm_Initialize()
Const nMin As Long = 300
Const nMax As Long = 650
Const nInc As Long = 10
Dim n As Long: n = Int((nMax - nMin) / nInc)
Dim arr() As Long: ReDim arr(0 To Int((nMax - nMin) / nInc))
Dim nCurr As Long: nCurr = nMin
Dim i As Long
For i = 0 To n
arr(i) = nCurr
nCurr = nCurr + nInc
Next i
ComboBox1.List = arr
End Sub
Private Sub ComboBox1_Change()
With ComboBox2
.Clear
.AddItem ComboBox1.Value - 5
.AddItem ComboBox1.Value - 15
End With
End Sub

Answer 2

Use simple math:
Private Sub UserForm_Initialize()
Dim i As Long
ComboBox1.Clear
For i = 300 To 650 Step 10
ComboBox1.AddItem CStr(i)
Next
End Sub
Private Sub ComboBox1_Change()
Application.EnableEvents = False
ComboBox2.Clear
Application.EnableEvents = True
Dim cb1Val
cb1Val = val(ComboBox1.Value)
If cb1Val > 0 Then
ComboBox2.AddItem CStr(cb1Val - 5)
ComboBox2.AddItem CStr(cb1Val - 15)
End If
End Sub