Hello, here is my Z axis design,

I'm thought it would be better to use ground steel plate so I can reduce the distance the spindle extends from the gantry. I plan to use 8mm ground steel plate with some thin side plate to add stiffness. I haven't built a cnc before though so have doubts if ground steel plate will be flat enough compared to aluminium tooling plate. I have done a bit of hand scraping in the past and don't mind spending time if the steel isn't good enough.

The supplier on one website states their ground steel plate is under 1mm per 1 meter, my Z plate is about 30cm long so 0.3mm would be the worst if I got a poor piece.
Maybe i'm over estimating the importance of saving around 12mm extension from the y gantry if i did use say 20mm aluminum. I intend to use the cnc for hardwood, perhaps a bit of nonferrous metal too. I'd like to be able to machine things 8cm high at most.

Any thoughts?
Thanks