Percent Calculation Assistance Needed

Question

What would be the best way, performance wise, to set a calculation that returns the percentage of a budget that is spent, even if the budget is set to 0?
Using just spend/budget as a custom item doesn't work, since sometimes the budget is 0.
As info, the spend is a summation of several invoices but the budget is not a summation. Should I be using the calculation of SUM(Spend), instead of using the default aggregate of SUM?
Budget is setup with a default aggregate of detail. Should that be changed?

Answer 1

Actually, you have two separate issues to deal with (ignoring the performance for now... we'll work on that later if necessary ;-):
1) two measures to compare, with different levels of aggregations and
2) divide by zero problem.
Savest way to solve 1) is to create a calculation with something like SUM(spend) OVER (PARTITION BY ... data-items-you-want-to-aggretate-on i.e. month, department ORDER BY same-items).
This makes sure that 'spend' always aggregates to the level you want. Disadvantage is that the calculation needs to be adjusted if you want another level of detail.
Now 2), you have to include an explicit "work around" for this. For example: CASE WHEN budget IS NOT NULL (or <> 0 - depends on contents of column budget) THEN (budget/sum(spend) - calculation you just created)/100 ELSE xxx. You decide how to handle the case where budget is zero, what should be after the ELSE. Should variance (result) be shown as 100% or 0% or ...?
Succes!