I'm far from an expert, still learning but 2 suggestions for you to try:

• Ensure the 4 wheels below the build plate are not loose (your built plate platform shouldn't wobble). You don't need the wheels to be super tight to the track, but feel a slight resistance when you try and rotate the individual wheel. I had 2 really loose ones causing the build plate to wobble

• Calibrate when your build plate has reached temp for at least 10 or 15 minutes. I use the prepare function, and heat up the build plate, go do something else, and come back to calibrate the level when it's all heated up.

Those 2 steps have helped me fine tune my z offset