I had this for setting tool height on my CNC Mill. I found it very useful for precisely setting the bed level and mapping out any high and low spots. http://www.edgetechnologyproducts.com/pro-touch-off-gage/ Not involved with the company in any way just a happy user of their products.
Yup, a dial indicator is a very useful tool for that.
We’ve recently purchased a few of these from Amazon. Not the best tool in the world, but more than adequate for helping us level the bed.
We are going to try and print a mount to hang it off the extruder to allow us to level the full bed easily.
I did that. See here: https://forum.lulzbot.com/t/adjustable-heat-bed-glass-mount/38/1
Just know that you’re adding weight to the extruder, which will bow the x-axis rails even more than they currently bow.
In case of using the solution proposed by the OP. Which will be the method to add that data in the setup?
Should I add an offset to the Z axis on the slicer ? Is this value something I can add to the marlin firmware perhaps ?