Name: AppBuilder 5 (B)
Rating: 4.9 (11 reviews)
Author: flowstate

Build your own datafield! No coding required.
The latest version of AppBuilder allows you to create your own calculated data field based on a simple math formula. If you want to get fancier, there's functions like IF and AVG. Your data can be recorded to the FIT file, so you can see the graph in Garmin Connect.

Full documentation and examples: http://ciq-appbuilder.blogspot.com/p/examples.html
Discussion thread: https://forums.garmin.com/developer/connect-iq/f/showcase/5428/data-field-appbuilder
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AppBuilder5 (B) is one of four clones, so you can have 2-4 formulas per activity (depending on device) and store up to 16 formulas. All my apps, including clones, available here:
https://apps.garmin.com/en-US/developer/8545c609-c030-4c0c-b3d8-4bc63ebdc901/apps?sort=name