How to use fetch and update to calculate current wage

I am new to deluge and I'm really struggling to figure out how to set up my creator app, particularly when it comes to referring to fields in other forms.

One example:

I have a form called Add_Employee which lists all basic employee info including Starting_Wage. I have a second form called Enter_Wage_Increase which uses a lookup field for Employee Name.

I want to have a field called Current_Wage that calculates the employee's new wage based on Starting_Wage + Amount_of_Increase.

For starters, I am not sure which form to put the Current_Wage field on, though I think I would like it to be on the Enter_Wage_Increase form, and secondly, no matter how many times I read the documentation on Fetch and Update Records, I can't get the Current_Wage field to work as I intend.

Can someone please help me figure out what the script should say?

I would love it if I could get so fancy as to have a condition where it would first check if any previous Increases have been entered for that Employee and if so, use the most recent Current_Wage + the new Amount_of_Increase to calculate a new Current_Wage, but I can't even get a very basic version of this to work so that seems really impossible.

I really need help to understand how this works because it is crucial to a lot of other things I want to add to my app going forward. Is there more documentation I can read, or tutorials that help with interactions between forms? I've already done the very basic deluge tutorial but that doesn't help with this at all.