Hello @CampaignerForLife
In your main wf define the variable like this:
vars.mainWFVar=1;
Master WF:
Then in the Sub-workflow you can use the same variables to add values to it:
vars.varfromMainWF = vars.mainWFVar + 5;
logInfo(vars.varfromMainWF)
Sub-workflow
And if you start the mast WF you see the updated added in the logs: