Query on application of Fees

I've read the details available on Binance regarding the fee shcedule etc, but I'm not clear on how and when the fees are applied and wondered if someone could help me understand?

I don't have enough BNB to cover fee's so I understand that I'm currently using 0.1000% fees

I want to make sure that I'm accounting for the fee's correctly when doing my totals, so that I've got an accurate tally, as opposed to making assumption and potentially resulting extra gains, that I don't actually have.

Currently I'm just using the costs and values that I enter when making buy / sell orders.

EG: I'll enter 20.00 GBP to purchase XRP, but when I check the order details, it instead shows 19.5129 as the "total"

Clearly I'm not using all the 20.00 GBP to purchase the XRP, as 27 XRP at 0.7227 is 19.5129 - So where is the fee coming from, whats covering it?

How can I properly account for the fee in a spreadsheet, where the market price, purchase price and XRP amount will vary?

Sorry, I'm really struggling to phrase this properly to indicate what I mean.

~Tk