Unfortunatly front & rear ARB siffness are linked. If you have a soft rear setup & it's stiff up front you'll get understeer, go for a stiff rear combination & softer at the front you'll get oversteer. It also depends on the tyres, springs & damper setings you have.
My recomendation is to start with the rear ARB set as siff as it'll go & if the car feels like it's wanting to oversteer back off a notch. This is safer than setting it to soft then going harder as you're much less likly to get caught out by oversteer problems.