You mention the use of a card to check the table calibration. I think card stock is too thick for that purpose. I currently use the 4 strips of paper method that someone here suggested and it works fantastic. I also weight down the build table with a steel bench block and push it down while calibrating. Also, check if the 4 table screw washers and the slots they are against have become distorted (concave) from over-tightening. Over-tightening the screws will create a depression that the table will migrate to when tightening, throwing off the calibration. If the washers have become concave, simply flip them over. The slots in the black frame may need to be filed a little. When calibrating, once the screws are tightened check that the paper strips at each corner are tight and won't slide out with a slight tug. If all 4 are tight, calibration is good. Do not further tighten the screws after the build table is off the arm and remember not to use any heavy pressure when removing models or cleaning resin off it. When handling it, I'm trying to get in the habit of holding the table itself and not the black frame (but old bad habits are hard to break!)
I recently added an extra set of washers and I hope that will prevent the washers from distorting over time.
So that's how I do it, I hope it helps.