How To Reset Auto Incrementing Textbox

I am working on a simple parts numbering system.

I am using a userform to allow the users to pick from two combobox to generate the first 3 digits on the part number (which are inserted into a textbox automatically), the final 4 digits are auto generated using the following code:

[Code].....

This is called when a button is pressed, this then checks the already existing parts sheet to see if the number exists, if it does then it will ask for another number, incremented with the previous code. This will continue until a unique number is found.

This is the code that does this:

[Code] .....

My issue is that if I reset the form to create a new part number which is in another catagory, I need the auto increment to reset to 0, so the new part number can be checked. At the moment it carries on incrementing from the point the form is reset. The same happens if the user decides to change a catogory (maybe for the first part in the catagory, should be ***-0001), if the last part was ***-0055, then it will assign the new part 0056, not 0001.

So how can I reset the auto increment to back to zero on for example a listbox / combobox change?