I have yet to do a pure MR. I've taken to tacking on mileage runs to trips I'm already planning to take, and looking at what I've started calling the mCPM (marginal CPM).

For example, this December I'm flying to LAS from SBA. Cheapest roundtrip would have been $360 on US, for 1420 miles when credited to UA (no 500 mile minimum). Instead, I'm flying SBA-SFO-CLE-MCI-CLE-SFO-LAS on UA to get there, leaving a day earlier, and returning LAS-PHX-SBA on US for $461. Total mileage is 7422, or 6002 additional miles for $101 more. So my mCPM is 1.68.

I did something similar last year to reach silver, and this year it'll be to reach gold. Last year is was an mCPM of 2.8.