Zero Clearance valve lash ?

Even if you adjust it to zero lash and go a little farther it won't hold the valve open because hydraulic lifters bleed down due to their design. If you do it right they will essentially keep the lash at zero. Setting them at zero lash and backed off 1/2 turn, like you say you have them now, puts lash in the valve train. Std. hydraulic lifters are not designed to operate with lash.