Actually, this was simpler than I was initially thinking. I got the answer I was after using this calculation:
Thanks Jim. I didn't know you could do this. I thought the round function only worked for decimals.
Thanks for the shout out
There are a number of Excel function that work in Tableau "Create Calculated Field" -
I just try typing them into the field to see what pops up - then go to the field list on the right of the frame