I think your “Plumbing Units” column is representing “Fixture Units”? Correct?
If you don’t use these fixture units where you are, then change the families to use a flow rate instead of a fixture unit. Then you don’t need dynamo at all really and your problem goes away. Actually it’d be even better since you can then use that flow to calculate/size your pipes.
If you are set on using Plumbing Units. What i’ve done for something similar is create a project parameter called “Converted Flow” and assign it to your pipes.
Then in dynamo hard code in this table you are using (if its a table that doesn’t change its better to hard code it in then reading from excel, IMO) then from dynamo, you will read in the fixture units in each of your pipes, convert that to a flow and assign it to that variable. to size the pipe you could do something similar using a hard coded table, flow rate and predefined sizes to set the size.
I’ve done this before, but you’ll see that the fittings dont cooperate very well and it takes some extra effort to resolve that problem.