Digital BW, The Print

What is the opinion of folks with regards to the Lab a and b values 
that should be strived for when producing a neutral curve?

I've been playing around for a good few days now trying to produce a 
neutral curve, and seem to be going around in circles at the moment, 
with values within plus or minus 0.5 for both Lab a and b; are these 
figures considered OK or should I continue with more iterations?

I'm using QTR with a 1290s and a UT3D inkset, and using an XRite Pulse 
for my readings,

Jamie.