You can go lower at the front that the rear. Omegas are close to as low as they can be at the rear as it is, before the diff constantly clouts speed bumps, drop kerbs, ramps in carparks etc.. I seem to think it's 30mm (MV6 being 20mm lower from standard anyway)
40 or maybe even 50mm
might be possible at the front. Don't quote, me, I'm on MV6 -20mm and that's enough for me, I'm the youngest old man in town!
Don't forget to factor in a possble quirk... when I replaced my standard with MV6 the back end went
up, and now no longer smacks a drop kerb where I park at work. Old, sagged , softer springs, vs newer, but 'lower' springs. You never know, this might even happen with you, too. What I was concerned would be 'too low' for certain town driving, in reality actually helped round-town practicality.