There are at least two types of this adjustment. I just checked my BG6000 (early AC motor version, similar to 4000) and 4004 (clearly lighter built DC motor version). There isn't any need to go below the leaf spring on either of these.
On the 4004, there is no tightening nut at all, you just turn the top nut which can be done with fingers if you push the spring down to release the pressure.
On the 6000, you loosen the top nut and then turn the entire adjustment pole, there's a slot for a screwdriver at the end of it.
I believe yours is the latter type (upper in the pic), Steve?
I always remove the dust lid as a first thing when working on these to avoid accidents