tiered commision cumulative threshold

Hi -
I'm trying to figure out ow to calculate tiered commisions on cummulative quarterly sales.


There are four commision tiers
0 - $60,000 = 0% commision
$60,001- $150,000 = 30% comission
$150,001- $210,00 = 40% commision
$210,001+ = 50% commision

I have variable sales made in each of 4 Quarters (Jun-Jul). I need to calculate the comission on cummulative sales basis each Qtr.

Eg.

Qtr 1

Client $16,500.00 $16,500.00
Client $16,500.00 $33,000.00
Client $35,000.00 $68,000.00

First Column is the invoice, second is cumulative invoice.

On the third sale, a commission of $2,400 is achieved (30%*8000), and so on. I start running into problems using the IF function and VLOOKUP when passing into different thresholds and apportioning commission rates between threshold rates as they are reached.

I can't seem to get my head around this.

Any help would be appreciated.

Thanks

Dan