[blykins]

The lash is determined by the cam grinder, which is based on the particular lobe design.

You can go either direction a certain amount to "fine tune" the engine. Closing the lash changes the effective duration of the cam, "fooling" it into thinking it's larger than what it is. Of course opening the lash up does the opposite.

If you had no clue on the cam or cam specs, I would start by getting a general feel of whether the cam is a roller or flat tappet, aggressive or lazy, etc. It should give you a ballpark.