How long is the runway? Are there any trees to clear?

Have you every seen an airport? They try to place them so that these are not issues.

The formula would alter with needs. So you have a great field, you get away with a little less. You are at 4000 ft, you need a little more.

Are we talking 1/3 scale here?
What are your flying sites requirements?

Do you really want the formula? Drag vs power = thrust / minimum flying speed.

Whenever I get a question like this, I point out that a 1 million pound 747 flies mach 0.85 with 1/4 million pounds of thrust. In your case 21/4 = 5.25 pounds of thrust to fly mach 0.85 at 35,000 feet (yea right).

So what is your real question? What motor to choose?