Machine ways warpage compensation with probekins

Hello everybody,
I start this post with a bit of a disclamer, i know that nothing beats a straight way and good machine geometry, but in the case of a DIY CNC, not everything can be achieved in the best way.

So, i was having problems machining a sort of flat face on a 400mm x 50mm surface the other day, it was coming out with a 0.7mm (0.027'') bow, and i couldn't for the life of me find where it was coming from, but after a thorough check of the linear rails of my DIY router i found out the same exact bow there.

Bit of context: i've "recently" casted a new epoxy granite base for my router (images can be found on instagram @radial_workshop), and after that i've sent the base to get the surfaces of the x axis and table milled, so to have both straight and parallel X Axis ways and a flat surface to reference everything to.

Having found that bow in the machine i started correcting it by loosening the Y axis rails and tapping them into position, as it was the worst axis in term of bow and out of straightness, and i managed to get within 0.25mm from end to end.

Now i started wandering whether or not LinuxCNC has some sort of compensation for not straight ways, and found ProbeKins.

The wiki for ProbeKins is pretty straight forward (wiki.linuxcnc.org/cgi-bin/wiki.pl?ProbeKins). I would probe the table with a fine mesh of probe points and follow the procedure to generate the mesh for ProbeKins.

BUT, being the table flat in relation to the ways, i would need to invert the Z axis correction, is there a way to do so?

If doing the same thing now, I think that I would bas it around external_offsets, not kinematics.

Taking a look inside external_offsets, i too think that i could be a better solution, but i'm now wandering if i can somehow program an external_offsets matrix that corresponds to the way's warpage like the correction .STL of ProbeKins