Sorry, I didn’t subscribe to this topic, for some reason.
You’ve got the whole machine on one plate of glass, and there’s no other plate in the middle with adjustment screws. That isn’t going to be easy to get right.
For one thing, when you’re printing in the middle and one end is too “low”, you’ve got to adjust that end to be short, but a) there are 4 adjustment points (the legs) and b) the location of the adjustment points are far away from where the problem is.
For another thing, as Ryan mentioned glass isn’t perfect, especially not at 1/4". Maybe if it was 1/2" it would be closer, but at these sizes, it’s almost definitely going to be rolling hills (from the extruder perspective.
The other small issue you’ll have is that level doesn’t really matter, and parallel planes isn’t really it either. The thing that has to be right is the distance from the plate to the XY “plane”. If this is different at different points, then you’ll get poor results. If the gantry sags in the middle, or there is a low spot b/c of one of the corners, then that will have to be fixed with manual levelling.
The “Auto level” really can mean a lot of things. The first systems just measured three points, and considered that a plane, and printed on that angled plane. Now, there are functions for meshes, so you can measure a point every 1" in a grid, and that mesh will be stored and used to always move the Z down to the plate at the distance that was measured. In between the points, the distance will be estimated. There is a limit to this, and the quality will improve if you get the bed as level as possible. I’m going to use my gut and say that if you’re within 0.5mm/100mm then the mesh leveling can compensate fine. The resulting prints will have the curvature of the build plate, but if it looks flat then it is. More important is that the bottom layer is a consistent thickness, which the mesh leveling will give you.
It makes sense that you need endstops for mesh leveling, because it’s measuring a Z for a specific X and Y. So if you don’t know the Z and Y in absolute terms for the machine, then it will pick the wrong Z.
For actually doing the probing, I use and inductive sensor on an aluminum build plate, but that’s not going to work for you. If you want it automated, where you probe every time you print, then a microswitch on a servo, or the BLTouch would be good.
You can use a z probe, which is just a wire attached to the hot end, and a wire attached to a spatula. When the hotend comes down, it will make contact with the spatula, and close the circuit, which looks like an endstop. Then you adjust for the height of the spatula. This method will work, especially if you just want to record the output and reuse it without probing for each print. The problem with this is, it’s not really a set of instructions, and I’m not going to figure out the details, so you’ll have some work on your own to figure it out. I can still answer some questions, but a lot of my answers come from guesses 
. When you figure it out, posting an instructable or something would be pretty nice.
There are good instructions for setting up the BLtouch probe though, so that’s definitely an easier route. Either method of mesh leveling will need you to be comfortable with arduino and the configuration. There are some pitfalls, so you’ll have to look at it as fun, or you’ll go crazy. I’m very comfortable with SW (I’m a SW engineer by trade) so I’m not really the best judge on the difficulty level.
If you want to go manual, then put another plate on top of the glass, and use three screws to adjust the plane. It will be nearly impossible to adjust with just the legs (IMO).
