You can't input a dimensionless numerical residual, but we can still manually override the convergence tolerance using the convergence criteria value or percentage which is effectively the same thing.
If you have a study setup you can go to Tools > Flow Simulation > Calculation control options.
For the particular goal in question you can set the goal criteria to manual and specify a convergence value (with units) or percentage (default is 3% delta).
The help button will take you to more useful information about the convergence delta criterion values mentioned above so that's the best place to start first!
I hope that helps.