Your question could have been better expressed which is why I think nobody has responded yet. Nevertheless, I understand what you're saying and I think I've spotted a problem with your algebra.
What is the -intercept of the function ?
I think you will find that it isn't as you'd hoped! This +10,001 is the culprit. I'm sure you can see a way to fix this: just solve for and you should be on your way.
Your approach in general is good, however, so your instincts were right.