What you're trying to do will not work. You're trying to pass an internal WFA command variable into the Return Parameters. Unfortunately, the workflow doesn't know what '$DiskLimitTotal' is. The $ on the variable will make the workflow think it is looking for a User Input to the workflow
Also, at this time, it's not really possible to pass command 'results' to other commands in the workflow... so I'm really not certain you'll be able to do what you're trying to do with the command.
That said, if you can find a way to pass the result into a command variable (say DiskLimitTotal ... no $), then the workflow will know that variable and it's value... and then you should be able to leverage the return parameters.
I'm not sure if it's possible, but if you do know the values of $DiskLimitExtendBy + $DiskLimitCurrent.... then you could leverage a WFA Function. You could pass those values into a Function... which would then in turn provide the value to a WFA Command variable (say 'DiskLimitTotal' - no $). At this point you should be able to use THAT variable in the Return Parameters and return the value you are looking for.