Cermag has 6.5mm thick ceramics, which is ever so slightly thicker than .25"
https://cermagmagnets.co.uk/ceramic-block---60mm-x-65-x-125mm-2126-p.asp
Thinking about my Invader magnets - it's been a while since I had them out - they may very well have been 0.2" instead of 0.25".I have discovered a few slightly different thicknesses in magnets I've taken out. If you wind up with something uneven between magnets of different thickness, you can use a shim between the mag and the baseplate to make it level before you screw it down. At least that's what I've done in the past and it seems to have worked.