Name: AppBuilder 5 (C)
Rating: 4.7 (7 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, like "Cadence / 2".  Also comes with conditional logic, alerts (beep and vibes), text display, functions (e.g. lapavg, avg, min, max, log). and the ability to record data to the FIT file so you can see the graph in Garmin Connect. 

You can store up to four formulas in the app and switch between them without a phone or PC.

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 (C) 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