On my Tri-Power which is somewhat similar I started with about .008 front to back (your Y?) and .018 left to right, your x. After 7 iterations I got it down to about .0015 total variation with the head about 1/2 way down. In doing this I noticed that it's important to lube the quad tubes and always move the head upward a little before locking to get a consistent head orientation.
I used forked shims from McMaster -Carr that fit around the bolts between the black plate and the lathe head. Also be sure to always add or subtract shims at the same time of the same thickness on adjacent bolts so the plate remains in the same plane. I torqed the bolts to 30 ft-lb after each adjustment. I noticed no binding in the quadra-lift tubes after finishing.
Did notice that the cheap bolts start twisting at about 35 ft-lb. I think the bolts should have been at least double this strength, but I don't trust the shallow tapped holes so will leave as is.
Haven't checked the lathe taper (or seen any problems in several parts) but I guess it would be better to straighten that out before tramming the head. I don't know if they have alignment pins in that connection or not. Every place around my x and y acme feed screws the alignment pins were grossly out of position and multiple attempts made I don't understand how the manufacturer can come so close to making a great product and screw it all up in the alignment and getting cheap at the wrong places