Try pushing the button when you are moving at a speed of 30mph or better.
Our older speedos kick into calibration mode when the button is pushed at a stop or at slow speeds, 1st gear etc.
The sequence has something to do with time/speed.
After I get fuel, I make sure I am rolling at cruising speed for a couple of minutes then I press and hold the button.
It always goes to the actual mileage and then switches to the trip (0.0) when it resets.
If you end up in calibration mode, I am pretty sure you can just ride it out. I know there have been a couple of times I have gotten there by mistake and have had to get the bike cruising and pushed the reset a few times until it got out of that mode.
Hope this helps.