OK for those that are not 100 % on valve train geometry, lets look close shall we.
Stock cam, stock block, stock heads, stock rockers, in other words no extra machine work like decking block or heads etc as those affect geometry as well.
The rocker does in fact like to work on the valve in a certain point and the closer to the centre of the tip of the valve the better! This is where ROLLER rockers have a wee bit more leeway!
hydraulic lifters have some cusion if you will for differences but it's minimal, and another reason we don't use adjusters, they are self adjusting, how much I don't know ask a more knowledgable person than I!
Now grind the cam's base circle which in effect raises the lift by comparision of toe to base and what that does is lower the pushrod in the block a wee bit canting the rocker back a bit too, now here is where the HYD lifter comes into play I think allowing the rocker to be forced back to the stock location.
If we had a solid lifter cam we would need to re-adjust the valves after installing a reground cam as it's done on the base circle and that clearance would increase, if we didn't it would become noisy!
Now lets cut the heads and deck...now we have the shortened the distance from pushrod cup on rocker to cup on lifter, this in turn presses on the valve sooner but wait, we have HYD lifters that take up that lil bit.
So the key is the HYD lifters and to NOT go beyond their capability to take up the slack or to handle the decreased clearances depending on what you do.
Every single thing we do in the engine that is machine work affects some other part down the line, and its how those parts intereact that is key to whether we have success or failure. Failure to recognize that there is an incredible amount of part interaction can lead to a BIG BAD KABOOM if you know what I mean, when tolerances stack up in a bad way.
I'm hoping I've cleared this up with a very broad approach to it. I'm not one for picky details unless requested but I could write a paper but wont cause there is more info on the net than I've forgotton in years.
You can research this further if you want more info!
Al