Perform math functions on custom fields

I was hoping I could find a way to get the custom field data, rather than just set it, within InfusionSoft. There’s apparently lots of ways to change and update custom field data. What’s missing is a way to get or read that data, then perform functions based on that data.

I can get custom field data with the API, then perform functions on my site or where ever, but can’t seem to find a way to do it in a campaign. I was hoping the decision diamond could at least fork campaigns based on custom field data, but even that seems missing.