I took a quick look at this one and I think the reason for this behaviour is that Tableau 8.2 and 9.0 treat that attr-calculation differently. Public is already using at least some elements of 9.0.
1 - ([Total minutes per site]/ATTR(([Days in month]*24*60)))
maybe you could use this :
1 - ([Total minutes per site]/AVG(([Days in month]*24*60)))
I tried your suggestion and it worked perfectly. Fantastic - thanks very much.
Having done more reading, it looks like changing ATTR to AVG works faster anyway, regardless of fixing the weird way it worked.
Thanks very much... Shaun
When you write "differently", what are you seeing?