Ryan's #2 has been my biggest challenge with chisels and my large belt sander.
Pressing down, finger in the center is where to start, but even then you might find you are still applying  bias to one side versus the other. This would be my guess with your chisel. 

When sharpening the bevel, I've found it helps if you have a fine point sharpie that you can mark a line (with a square) near to the tip. This gives your eye a reference to see if you are going square or starting to go astray. Moving the finger pressure to the long side and adjusting will get you back on track.

Once the bevel is flat and square I like to do the micro bevel. Makes it quick and easy to get a perfect edge when you need to refresh it.

I don't have any power tools for fine sharpening so my MKII is the go-to for the bevel and then my finger placement can adjust to the micro bevel during a refresh .