Insuficient lifter pre-load will cause excessive valve train noise. Excessive lifter preload will cause the motor to idle rough or have low manifold vacuum and can lead to severe engine damage. It is critical to engine efficiency and to the service life of the valve train (camshaft, lifters, pushrods, valve springs, etc) for the lifters to have the proper amount of preload. On any hydraulic lifter camshaft, the ideal pre-load shuold be 0.030" A variance of +-0.010" is acceptable

With adjustable rocker arms assemble the entire valve train. Be sure the pushrods are seated properly into the lifter and rocker arm seats. Turn the engine over by hand in the direction of rotation until the exhaust pushrod just begins to move upward to open the valve. You are now ready to adjust the INTAKE  of the same cylinder. Carefully tighten the adjusting nut on the intake rocker while spinning the pushrod with your finger tips. You will feel a slight resistance in the pushrod when you have taken up all the clearance. This is refered to as "zero lash". BE CAREFULL with misjudging 'zero lash' for the bottom of the lifter adjustment with new lifters. Without oil in the new lifter the resistance at zero lash will be very minor. While we are on the subject, DO NOT attempt to pump the lifter up before installing them. Many of the high performance lifters will pump up and not bleed oil off causing the valve to be held off it's seat.

Now that you have established zero lash you can set the lifter preload. Most aftermarket lifter manufacturers suggest 1/2 turn beyond zero lash to set the proper (0.030") preload. I personally have used 1/4 to 1/3 turn for motors that will see higher rpm's frequently, and 1/2 to 2/3 turn for street motors that seldom see the high side of 5000 rpm. Some cam manufacturers claim more usable RPM potential with the 1/4 to 1/3 turn. Because of the forgiving nature of hydraulic lifters you can set the pre-load hot or cold, on the engine stand or in the car.  The very purpose of the hydraulic lifter is to compensate for small errors in valve train adjustment so forget the old days of the the hot  adjustment with the motor running unless you suspect you errored in your initial adjustment.

Now that you have the preload set for the intake, once again rotate the motor in it's direction of running rotation until the intake pushrod comes all the way up and almost all the way back down.  Continue rotating the motor gently till you can just turn the intake pushrod. Now establish zero lash on the EXHAUST rocker and set your desired preload. With one cylinder complete continue to set pre-load on the remaining cylinders.

Do it right the first time during engine assembly and you may never have to adjust the rockers again.