Variable Percentage Rate

Hello, all.

Here's the situation:

$0 - $180,000 investors get 75%
$180,000 - $450,000 investors get 50%
$450,000 - $900,000 investors get 0%
$900,000+ investors get 30%


Based on this link, I was able to come up with this formula, where D8 is gross revenue:

=SUMPRODUCT(--(D8>{0;180000;450000;900000}), (D8-{0;180000;450000;900000}), {0.75;-0.25;-0.75;0.3})

For example, if you have $250,000 gross revenue, investors receive $170,000.

Where this all breaks down is after $450,000. Any ideas?

All help will be greatly appreciated.

Thanks much.