In general, Quick Base records don't know about data on other records without a relationship in place.
The simplest setup I can think of to accomplish what you outlined here is to create a relationship from your Units table to itself. You could then have a summary field in the Process to Unit relationship to show what the latest Record ID# is, setup that in a lookup field back to the Units table, a snapshot to capture what that value is as a new Unit is added, and an Automation to populate the reference field in the relationship I proposed to link them together.
With that relationship in place, you can add the Actual Units and Forecasted Units fields as lookup fields so that you can easily see what the previous month's metrics were. Your current formula could then be based on those details like this:
If([Previous Unit - Actual Units]=0,[Previous Unit - Forecasted Units],[Previous Unit - Actual Units])*[Related Process - Growth Rate]
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Eric Mohlman
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