I'm making an email under script maker, and I would like to change the font to arial narrow. But how do i do that? I know that i have to use some calculation...

You cannot.

FMP sends plain text -- no formatting, just data. The final appearance of the message is determined by the font settings of the recipient's mail program. Out of your control.

If you want to send "rich" or "html" messages you'll need to use a third-party plug-in like mail.it or SMTPit.