I need help creating a formula to calculate commissions on revenue
Tier 1: 0 -1,680,000 = 3.5%
Tier 2: 1,680,001 -3,360,000 = 4%
Tier 3: 3,360,001+ =5%

Once the revenue reaches a particular tier all commissions are paid at that level. For example asssume current revenue is at $1M and I close a deal for $2M, then the first $680K of that $2M will be paid at 3.5% (Tier 1) and the remaining $1,320,000 will be paid at the Tier 2 level of 4%