The plugin comes with several preconfigured ways to calculate prices but if you want to create your own calculation you can use the price type “Formula”
Selecting that option will open the formula builder
In here you can put your formula, most of the times your formula will just be an arithmetical operation but you can do a lot more things like adding conditions (explained in another tutorial).
You can click on the add field button to add one field to your formula
The add function button will let you add useful operations like rounding a number
Every time you do a modification to the formula the plugin will automatically validate it and will let you know if there is any error with it