90 degrees at zero lift gives you fastest off the seat valve action and least lift at max lift.

90 degrees at open gives you most total lift and softest touchdown on the seat but it is hard to keep the valve from losing contact with the rocker tip.

90 degrees at mid lift (or somewhere close depending on who you talk to) gives you the best of everything.

Anywhere except 90 degrees rocker/valve relation will give less than max ratio. Don't forget that if you have a roler tip than the (theoretical) tip of the valve is actually going to be the center of the roller.