Return Column Header When Value Meets Condition

The challenge I am faced with is very straight forward.

I have a total budget of 300 widgets for a calendar year.

Each month I expend a certain number of widgets.

I need a formula that returns the first date when expended widgets are greater than or equal to 10% of my budget...in this case 30 or more widgets. The answer in the example below is Apr-12, but how do I structure a formula to automate the calculation?

Any help would be appreciated.

Jan-12	Feb-12	Mar-12	Apr-12	May-12	Jun-12	Jul-12	Aug-12	Sep-12	Oct-12	Nov-12	Dec-12
5	10	10	15	70	40	50	30	70	30	40	30

Pl see attached file with different formulas.