I did something similar using a 1350mm belt and four idlers.
I wanted to keep the center of the base plate wide open so that the wiring to the bed heater could coil neatly there (and it absolutely does).
IdlerModifications4_layout.pdf (6.7 KB)
It was extremely simple to install and cost very little to do.
I installed it while initially building the printer as mentioned and so as a result I have literally never had any issues with side-to-side bed leveling due to the two motors.
And then later on I doubled the corner guide rod bearings and now I don't have any issues with front-to-rear leveling either.
Seriously, it works like a charm and is cheap and simple.
Specifically to the point @Wyvern made, yes the bed support plate is leveled.
To level the printing surface more is required.
To that- the only problem I am currently experiencing in regard to leveling is bowing of the mirror tile I am using for a print surface.
It is due to the edge clamping and variations in temperature across its surface.
It isn't really a problem for any printed object smaller than maybe 100x100 mm, but for anything larger the edges start to become a bit dicey requiring the use of a raft (which I find wasteful).
So the center runs maybe 0.05-0.10 mm or so (my best guess) higher than the edges at ~100C despite the mirror tile being dead flat at room temperature.
I designed an optical sensor to work with the mirror tile, and it actually worked.
However it would easily sense the height variations due to uneven hair spray or ABS juice, wiping out any benefit it provided.
I had hoped the infrared beam I was using would punch through those films to the underlying reflective surface but it didn't quite work out that way.
If anyone would like to play around with it, I will gladly post the design files (an stl file for the body and a schematic).
So now I have purchased a BL Touch sensor and am about to install it this weekend.
Fascinating gadget, and I love the design of it with one exception.
It uses a hall-effect sensor for the extended limit.
A precision contact setup (a ball or cone and two pins) would be far, far more precise and sensitive, perhaps down to +/-0.0001 mm.
This principle has been used for decades for ultra high precision measuring equipment.
But it is what it is and I have high hopes for it.
Perhaps after I get comfortable with it I will design my own version of it with a ball and pin switch just for kicks.