The Michell VTA adjuster available from musicdirect.com works pretty well if your arm doesn't have and adjuster.
http://www.musicdirect.com/p-7336-michell-rega-vta-adjuster-for-michell-turntables.aspx
It should make a difference you can hear. Also have you checked the cartridge alignment with an alignment gauge?
I think the green slider you mentioned is the anti-skate adjustment which should be set to the cartridge tracking weight/force or slightly lower.
http://www.musicdirect.com/p-7336-michell-rega-vta-adjuster-for-michell-turntables.aspx
It should make a difference you can hear. Also have you checked the cartridge alignment with an alignment gauge?
I think the green slider you mentioned is the anti-skate adjustment which should be set to the cartridge tracking weight/force or slightly lower.