Unexpected String Results - vba

Question

I have the following code to check values entered into two input boxes, if both values are zero then the MsgBox should display "Stop!" (I will change this later to exiting the sub but I am using a MsgBox for testing)
From testing I've seen these results:
A zero in both strings produces the expected message box.
A non zero in the first string followed by any non zero value in the second string does nothing (as expected).
A zero in the first string followed by a second string value equal to or greater than 10 produces the message box (unexpected).
I've also noticed that if the second string is 6-9 it is displayed as x.00000000000001%. I think this is a floating point issue and could be related? This behaviour occurs without the IF... InStr function too.
Option Explicit
Sub Models()
Dim MinPer As String, MaxPer As String, Frmula As String
Dim Data As Worksheet, Results As Worksheet
Set Data = Sheets("Data")
Set Results = Sheets("Results")
Application.ScreenUpdating = False
MinPer = 1 - InputBox("Enter Minimum Threshold Percentage, do not include the % symbol", _
"Minimum?") / 100
MaxPer = 1 + InputBox("Enter Maximum Threshold Percentage, do not include the % symbol", _
"Maximum?") / 100
If (InStr(MinPer, "0") = 0) And (InStr(MaxPer, "0") = 0) Then
MsgBox "STOP!"
End If
' Remainder of code...
This is the most interesting problem I've come across so far in VBA and welcome any discussion about it.
Edit: I use this code to display on screen the paramaters for the end-user to see. Hence how I noticed the .00000000001% issue:
.Range("D2").Value = "Min is " & 100 - MinPer * 100 & "%"
.Range("D3").Value = "Max is " & MaxPer * 100 - 100 & "%"

Answer 1

Two things
1) Declare MinPer, MaxPer as Long or a Double and not a String as you are storing outputs from a calculation
2) Don't directly use the InputBox in the calculations. Store them in a variable and then if the input is valid then use them in the calculation
Dim MinPer As Double, MaxPer As Double, Frmula As String
Dim Data As Worksheet, Results As Worksheet
Dim n1 As Long, n2 As Long
Set Data = Sheets("Data")
Set Results = Sheets("Results")
Application.ScreenUpdating = False
On Error Resume Next
n1 = Application.InputBox(Prompt:="Enter Minimum Threshold Percentage, do not include the % symbol", _
Title:="Minimum?", Type:=1)
On Error GoTo 0
If n1 = False Then
MsgBox "User cancelled"
Exit Sub
End If
On Error Resume Next
n2 = Application.InputBox(Prompt:="Enter Maximum Threshold Percentage, do not include the % symbol", _
Title:="Maximum?", Type:=1)
On Error GoTo 0
If n2 = False Then
MsgBox "User cancelled"
Exit Sub
End If
If n1 = 0 And n2 = 0 Then
MsgBox "STOP!"
End If
MinPer = 1 - (Val(n1) / 100)
MaxPer = 1 + (Val(n2) / 100)

Answer 2

This is because the number "10" has a "0" in the string (second character) so both evaluate to true.
Try this instead:
If (MinPer = "0") And (MaxPer = "0") Then
MsgBox "STOP!"
End If
For additional control save the user input (MinPer , MaxPer) and THEN text them for validity before performing nay mathematical operations on them.

Answer 3

InStr(MinPer, "0") is just checking to see whether the string contains a zero
character.
You need to convert the string value to an integer. Use the IsNumeric and CInt functions
to do that. See this URL:
vba convert string to int if string is a number
Dim minPerINT as Integer
Dim maxPerINT as Integer
If IsNumeric(minPer) Then
minPerINT = CInt(minPer)
Else
minPerINT = 0
End If
If IsNumeric(maxPer) Then
maxPerINT = CInt(maxPer)
Else
maxPerINT = 0
End If
If minPerINT = 0 and maxPerINT=0 Then
MsgBox "STOP!"
End If
Depending on what data can be entered It may also be a good idea to check if the length
of the data is zero using the len() function.