HTML Calculations?

Just wondering if the html will do calculations like
{BTRENT} / {BNNGHTS} * .80

I tried a few things but it doesn't seem to recognize any of the html code for calculations that are suggested in web searches I found.

Trying to automate my discount offered to booked guests for orphan nights adjacent to their stay to reduce the number of emails back and forth. Thanks for your help!

There is another way of doing it. You create a trigger X days before guest's arrival if you have orphan night before/after their reservation. And trigger will send them email like this:

Hi {CFIRST},

I just noticed that the {BADJOPEN} your reservation is still open.

Would you like to stay an additional night for 25% off? (plus applicable tax).  If you are interested, please reply to this email.

Thank you for choosing {PDISPNAME}!

If they agree, you just do manual calculation of how much it will be.

I am looking for a way to do this for final payment. I want to offer a two percent discount for check payments, and want to calculate the amount for them in the email.

Has anyone been able to automate this?