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?

Probekins wasn't entirely succesful, so might not be the best way to achieve what you want.

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

I don't think it matters what is flat and what is bowed, probekins will apply the correction in the right direction. The effect of both is identical.

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

UPDATE:
I wrote a script that reads a .csv file containing a x, y, z comp values, and outputs the z comp values based on x y coordinates.

Now, i've compiled the .c file, tryied to start linuxCNC, it loads the matrix, than crashes with the following errors:
HAL: ERROR: data_ptr_addr not in shared memory
/usr/bin/rtapi_app exited without becoming ready 
insmod for external_offsets failed, returned -1

I've mostly got how the external offsets works, now i only need to implement it with the matrix and get it to work.

Other point: reading online i saw that most of the eoffsets installations switch the compensation off while in machine off, why is that? Is it because it can cause problems with homing?

Does a basic guide on configuring eoffsets exist?