Adding two numbers together using an automation instead of just being text

That works in a formula-numeric field, but my problem Is that I am using automation like this

If you build a formula; then that will generate a value for THAT record.  If you are then trying to push THAT RECORD's output value into a NEW record, then encoding the formula would make sense.  Otherwise, if the new record should have the unique value of the [Record ID#] plus 99,00000 then simply build a formula and you do not need to encode anything anywhere, since the [Record ID#] is unique to each record regardless of deletions.

Ok, so I got it working. Since Part # is a text field, and I only want it to assign a Part # if the user puts "todo" in the Part # box. I made another field that, like you said, is a Formula-Numeric field called "This_Part#". It then does the calculation. Then the automation will check every time an "Inventory" is added or modified. If it was then it checks if Part # = todo. If that checks out it then sets Part # to "This_Part#" This is how I did what you said. Is this a good way of doing it or is their a better way?

What is the automation doing?  Is the automation putting the number that you generate (99,0000 + Record ID#) into a text field called [Part #] when the checkbox is checked?

So, when you make (or edit) an inventory, you can set a part #. What my Automation does, is when you put "todo" in for the part #,  it will set the part # to This_Part#( This_Part# is the 99000000 + [Record #ID] Thing. It is currently set so when you first create inventory, it will default to "todo" Then if you don't change it to your own part#, it will instead set a part# for it automatically.

Perhaps instead of doing it this way, you should use a [Part #] formula with an override Part # entry field.

[Part #] (formula-text field)
Formula:
If([Part # override] <>"", [Part # Override],  
990000+[Record ID#])

This way you avoid the automation, and users can use the [Part # override] field to enter manual Part #s without having to trigger an automation at all.

The way you have it designed, when you create the records, the automation is going to do what a Formula will do with no effort.  Users can then use another field to override the default-part number entered with their custom value.  This also makes it easier to see which items have customized part numbers versus not (over time).

Do I make a new field called [Part #] that is a formula-text field, or can I change the [Part #] that I currently have to formula-text field. (It is currently just a text field)

Is there any custom-data in your current [Part #] field?  If so, make that field the "override" field; and build a separate formula field to calculate the output if that field is empty.  In fact; I would do it that way to be safe.

What exactly is an "Override" field, is something I click on when making a field, or is it code in the formula, or something else?

It is simply a data-entry field.  You cannot edit formula fields; so for a formula to display something non-calculated; it must reference another field either generated automatically or entered by a User.  I call it an "override" field simply because we want the automated-value to be "overridden" with a data value entered by a user.