Time between steps in a process (Calculating formula just once)

Hello All,

I’m implementing a base that will be used to manage a process. As any process it has multiple steps and I want to identify which of them are bottlenecks. In order to do that, I created a Single Select field (Process_Step) that represents the current step of the process and other n fields that will hold the time that the process reached each step. For instance, S1 will hold the date and time when the process moved to step 1, S2 to step 2, and so on.

I implemented it as follows:

Field Last Updated Time = LAST_MODIFIED_TIME({Funnel})
Field S1 = IF({Process_Step} = "Step_1", IF({Origem} = "Maping", {Last Updated Time}, BLANK()))

The field Last Updated Time contains information about when the process moved one step and S1 will hold that value if it has entered the first step. The problem is, every time the process moves to further steps the formula is recalculated and S1 becomes BLANK().

Any suggestions about how to make it work? I was thinking about using the Airtable Python Wrapper, but if it’s possible to keep it simple I would prefer.