How do I perform a calculation (in a formula field) that uses a value from a field from another record in the same table?

Waldy,

I did some experimenting and came up with this.



This is the formula I used for the Ratio field 
[Widget 1 Installed Diameter]/[Related Widget Installed Diameter]

I hope this helps get you started. To Mark's point, this solution requires the end user to manually select the related record, which could become cumbersome or difficult.  You will want to make changes to the Record Picker to make it easier to chose the correct related record. 

I would look for a way to automate the creating the relationship, or at a minimum make the related Widget(record) field a conditional drop down.