Formula Help - Tiered Annual Commission Structure by Monthly Payout

Hello,
I am working on a Commission Calculator for my sales team, and I am hitting a wall on the formula for a Monthly Payout based on an Annual Commission Structure. I'm sure this is a simple fix for one of you Excel Gurus. Please help!

Annual Sales/Commission Rate:
0-100,000 = 10%
100,001-200,000 = 11%
200,001-300,000 = 21%
300,001-350,000 = 25%
350,001- 400,000 = 30%
400,001 + = 35%

Example (July - June Fiscal):
John sells 50,000 in July, he gets 10% commission in July
John sells 75,000 in August, he gets 10% on 50,000 and 11% on the remaining 25,000 for August
John sells 10,000 in September, he gets 11% on that 10,000 in September
John sells 75,000 in October, he gets 11% on 65,000 and 21% on the remaining 10,000 in October
John sells 10,000 in November, he gets 21% on that 10,000 in November

and so on......

I have attached my file if someone would be so kind to help with this formula for Row 14 (Columns B-M). The Rep will manually add their monthly sales in Row 11 (Disregard Row 12)

THANK YOU IN ADVANCE!
K8JXN