Bonus Calculation

I am tryining to create a spreadsheet which will give me the percentage of bonus which needs to be paid out, based on sales results. I have only been given 4 targets. 0 bonus if sales are 3,431,960 . The bonus goes from 0 to 25% as sales rise to $3,502,000, 125% bonus up to sales of $4,140,000. The actual sales were $3,943,450.
So the % of bonus they need to pay out lies between 25% and 125%. I used the same formulas they used last year (which is attached). I'm not sure how this formula can calculate the linear progression (wrong terminology?) of the bonus payout %. Does anyone have another method of creating this calculation so I can prove the math? I don't pretend to be an excel whiz so please try to take it easy on me. TX