Calculations / Automation / Auto number not saving to field?

Hi, 

I'm having a little trouble getting a automation flow to update a field so i'm not sure if there's a better way of doing this ..

I have a 'Projects' app, and an 'Revisions'  app in a different workspace.

'Projects' references the 'Revisions' app and one project can have many revisions. 

I am trying to auto-number the revisions with each new revision created Project by Project basis, rather than auto numbering the whole 'Revisions' app. 

e.g. 'Project A' might have revisions numbered 1-3,  'Project B' might have different revisions also numbered '1-3'

- if that makes sense 

so far ...

I have a field in the 'Project' app that auto increments 'CALC_NextRevNo'

this is referenced in the 'Revisions' app in a calculation field as - 

calc field: 'GET_RevNo'

Number(@All of CALC_NextRevNo);

the revision number passes, and appears correctly in the calculation preview - e.g. "4"

if i set an Automation (pictured) to copy the value of the 'Revisions' 'GetRevNo' to a text (or number) field 'Revision number' it doesn't copy / store it.

I've tried it as numbers and strings, and converting them around etc but no luck so far.

The flow is triggered when a new 'Revision' is created and can update other fields such as a category field. It doesn't seem to report any errors either.

Is there an easier way to increment a number for each 'revision' added to a single Project, or am i missing something in automation?

thanks,

Pete.