So from the looks of it, you have a bit of work to do. You need a widget, a text variable to display the “percentage”, a float variable to hold the value of the “percentage”, a method to increase the “float” variable over time and finally you need to link the float and text output variables. I made a tutorial for a racing game timer that has all the basic things you will need to implement to get something working. You will need to modify the code a bit if you only want it to come up with an “inventory” (also another widget), and I used an integer variable so my numbers are whole not decimal but that is a simple switch from integer to float variables. Anyway, check this out, try and get a basic system working first and then I can help you go from there to get specifically what you want.
