yes you can, not with the built-in action sets though. There are a bunch of PowerShell actions out there that allow you to execute PowerShell commands in your workflows.
I know PowerShell is not a calculation software but it's capable to execute the calculations you're searching for besides a lot of other useful integrational stuff that you can do in it.
Maybe this is an option for you.
If you already have those var values in a list or write to another list, you could build a calculated column using an Excel-like formula. That's a bit simpler than XSLT that Manfred Lauer suggests. But you must protect the list from inadvertent formula modifications.
You can do whatever calculation you want but you have to put some extra developments efforts but will not encourage to use huge calculation in workflows it will decree your workflows performance batter you can use calculated column in SharePoint and use in Nintex
Yes you can but its better do use a calculated column. I had the experience where it was slowing down the farm on our servers.