Good morning - I would like to see your book and the data -
there are 2 types of nulls - one with just a data value that is null and another where there is not a record in the data set - when you split across the rows you may be running into the second type - here is something to try -
IFNULL(lookup(SUM([Number of Records]) / TOTAL(SUM([Number of Records])),0),1)
You may need to play with the syntax a bit - I don't have a data set to check against
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