What is the correct treadmill incline to imitate outdoor running?

Usually 1% to balance out pace.

I like to take advantage of the lack of wind resistance and run at a faster pace at a lower effort. Especially with tempo running and what not, I feel like it helps build turnover.

It depends. Keep in mind that neither the displayed incline nor speed is likely accurate. Wind resistance is also exponential, so it depends on how fast you're going. The cushioning of treadmills can also reduce the effort required to run at a particular pace.

But at workout paces (10 mph or above), 1% gives you a reasonable approximation.

Would a 1% incline also be accurate enough for an average run speed of 8.3 mph?