I think the levelling business is to take the worst of the twist out of the bed and bring it close to its original just-ground level of accuracy. However, that isn't going to guarantee the accuracy that the lathe should be able to achieve. Once it's levelled as accurately as you can manage, then you switch to the "machining" test - the test bar machined at both ends. As long as the bed is not worn, there shouldn't be any need for grinding. That's already been done and you are just tweaking out any slight twist that develops in the casting as it ages and stresses release. The instruction manual for my lathe - cast iron integral stand, etc - recommends the "machining" test after setting up with a level, and then rechecking over the next few months. I think the BH 600 will be second-hand (although rather newer than my lathe) so it's likely to be fairly stable by now. I would set it up with a level as close as you can get and then fine-tune by machining. I can't see any point in using self-levelling techniques as they still won't give you the ultimate accuracy although it will certainly help overcome the problems of an uneven floor when first installing.