How to add a discount (percent %) field?

How to add a discount (percent %) field?

My particular case it’s regarding the “Opportunities” module. I’ve created a subform to calculate the value of the opportunity, and now I’d like to include a discount field in the form of a percentageHowever, no matter how or where I add the "percent" field, it always treats the value as a regular number. Attached is an example of how it currently looks.

As you can see, the Disc. (%) field is meant to be a percentage, but it’s still calculated as a number. So instead of:(List price: 80,000 * Quantity: 2) – (Disc. (%): 100%) = 0 EUR

It becomes:
(List price: 80,000 * Quantity: 2) – (Disc. (%): 100) = 159,900 EUR

I’ve tried placing the field in other sections and referencing it via formula, but it’s always interpreted as a number instead of a percentage. 
Is this functionality currently broken, or is there a trick to make this work?

I really need a quick way to apply varying discounts to opportunities and calculate a total, any guidance would be much appreciated
😊