Using rangesum() instead of the addition operator + should be more stable when handling NULL as operands:
sum([Initial Value])/sum([Initial Value]),
above(sum([One Month])/sum([Initial Value])),
above(sum([Two Months])/sum([Initial Value]),2),
above(sum([Three Months])/sum([Initial Value]),3),
above(sum([Four Months])/sum([Initial Value]),4)
I am actually using rangesum() in the application already. Unfortunately, it does not solve the problem of QlikView needing to populate the dimension field for the earlier months in the chart object in order to be able to calculate the equation.
To put it another way, having a NULL value there is actually the DESIRED result, as when there is not enough data for me to get all five elements in the equation, I do not want it to return a result, as that will confuse users. (If the equation was applied to the 'Jan 2013' row and NULL values were assigned a value of 0, the equation would return (10/10+0+0+0+0) = 1, which would suggest very poor customer retention in that month - something that simply isn't true.)