In general you only adjust valves on a 2.1 after doing a rebuild, as the hydraulic lifters self-adjust for normal wear. I've had them go 150,000 without adjusting, but most are probably passed that by now.
Because they go so long without adjusting, the adjusting screws develop a large flat where they contact the valve. Before adjusting, get new screws (10 mm) from VW if possible. There are aftermarket screws, but all I've found are too soft compared to the hardened original ones.
First key is to properly fill and bleed the lifters before assembly. If there is air in the lifters the springs will compress easily and make it hard to tell when the play is taken up. Then, after assembly, make sure the pushrods are properly seated in the lifters and arms.
As each cylinder is pulled to TDC with the valves closed (remember, there are 2 TDCs: one with valves closed (compression stroke) and one with valves open), adjust the screws so they just touch the valve stem with the free-play taken up. Then adjust them in (compressing the lifter) another 1/2 to 1 1/2 turns. If everything is really bled, this may open the valve a bit until the lifter bleeds excess oil and adjusts. Then tighten the lock nut. (VW originally said compress 2 turns, but if it starts out with a bit of air that can over-compress.)
Remember, it's not unusual for a 2.1 to have valve clatter when the oil drains out of a lifter, and that can continue for some time until it fills and gets rid of air bubbles. That takes getting the oil hot and thin. There is much less trouble with clattering lifters when using a thinner synthetic, such as a 5W40, if the engine wear will give adequate oil pressure.
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