I saw this question today on Trailhead, where the poster wanted to replicate something they had in an older system.

In this example, Existing Approvals is a static field. This individual was looking to emulate the same format in Flow, where, if Increase Requested goes up or down, the New Total row reflects the new total of the first two rows. Simultaneously, if New Totals is directly adjusted, the value in Increase Requested should scale up or down to match.

Reactive Screen Flows to the rescue, as this ask is not so challenging in Flow! Let's start by creating a Screen Flow and adding 3 currency fields. 1 for our static field, and 2 more to represent Increase Requested and New Totals:

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I've added a default value of $20000 in field one, but ideally, you would use the Default Value field to dynamically pull in whichever starting static value you are looking for

Next, we need to make 2 formulas; one for Increase Requested, and one for New Totals:

{!New_Totals}-{!static_number_Existing_Approvals}

IF(ISBLANK({!Increase_Requested}), {!static_number_Existing_Approvals}, {!static_number_Existing_Approvals}+{!Increase_Requested})

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Since the default value of Increase Requested will start empty/null, we want to make sure we only show the Sum of the fields when Increase Requested receives a value

Now we simply assign those formula fields to their respective fields:

Now we can test!

As you can see in the GIF above, the component does everything that the component we modeled this off of did. Reactive Screens has given us the ability to write these directly as formulas, rather than leaning on Lightning Web Components or other custom development. Neat!

Using Screen Flow Reactivity to make calculations where fields impact other fields in Salesforce