Calculate increasing commission rate

I need to create a commission tracking spread sheet. Our commission structure is a percentage of gross profits over 30%. My sales persons receive 1/2% on the sale if the GP is calculated at over 30%. That part of the formula is easy. But there is a second half that's calculated when the project ends based on how the project actually fares. If it closes at 30% then the sales person has already been bonused and nothing more needs to happen. If it's under 30%, his bonus needs to be paid back. However, if the job finishes at 35% or 39%, more commissions are paid. Instead of this "step" method, I would like to switch to an structure that increases with each percentage point. Below is the structure I am using now:

½% on all sales. Must have minimum bid Gross Profit of 30% or approval from supervisor to submit bid with a forecast of less than 30% GP.

When the job closes out and final payment made, Ray will receive the following bonus based on the profit above the 30% minimum.

½% additional bonus on jobs with a profit margin above 35.0%
½% cumulative additional bonus on jobs with profit above 39.0%

All sales above $1,000,000 during year one will have an additional ½% sales bonus awarded at contract signing. All estimates must be approved by the Construction Manager.

All jobs with less than 30% GP or less than the forecast GP if estimate is put in at less than 30% will have a claw back of ½% on the monthly commission checks.

Estimated

Actual

Job Name

Date of Sale

Contract Amount

Estimated Cost

Estimated Gross Profit

Estimated GP %

Estimated Bonus (.5%)

Date Paid

Final Contract Amount after change orders

Actual Cost

Actual Gross Profit

Actual GP %

Bonus for 30% (already paid)

Bonus for 35%

Bonus for 39%

Paid to Date

Total Bonus

Final Bonus

Date Paid

Lakeshore Apartments

12/10/11

$57,350

$39,513

$17,837

31.1%

$286.75

01/01/12

$65,350

$42,000

$23,350

35.7%

$326.75

$326.75

$0

$286.75

$653.50

$366.75

07/01/12

As you can see from this table, my employee received an addition .5% for closing the job at over 35% GP. But he got nothing for anything OVER that 35%. If this job closed at 38.9% he would still get nothing. I need to calculate a gradual increase from 1/2% bonus at 30% GP up to an as of yet undetermined about at say 100% GP.

The basic formula I am using now is this: =IF(F8>30%, (C8*0.005), 0)

Is all of this about clear as mud?

No. I am a department head looking for a way to standardize and increase the commissions my company pays my sales force. Using our current method, my guys get no increase between 35% and 39%. I would like to go to a "sloped" method rather than the current "stepped" method we are using.

So I want to pay a .5% commission for all sales over 30% gross profit. Then increase that commission rate by .01% for every .1% increase on the gross profit.