What is the formula for a percentage?

Hello, do I need to apply a 3% discount to a subtotal so that I can get the total?

I have tried multiplying the subtotal by 3 and dividing it by 100, but the field does not show me the result. Why is it that I would like to know what is the correct way to do it with this plugin?

My formula looks something like this… Subtotal * 3/100 = Total

I await your reply, thank you!