I'm in somewhat the same boat

If given his values, my company would apply the logic that demand per month taken from initial inventory results in FC.

So given 100 units in sales per month, you'd need to take your starting inventory from Month A (350), subtract 100, then move another month out, etc..until you hit a potential negative. In this case the return we'd be looking for would be 3.5 months on hand, but the formula escapes me as well.

350-100 = 1 mo
250-100 = 2 mo
150-100 = 3 mo
50 -100 = 0.5 mo

Giving a total of 3.5 months