Nm9 has it about right. There is no percentage, it depends on the pushrod angles which change throughout the lifter and rocker travels. The cam base circle will change the starting point angle, but the top of the lobe will produce about the same angle with a given lifter. The lifter seat height will have an effect too.
Now consider the crappy rocker designs out there, and that will also affect the angles. Add in my geometry correction on the LA stuff, and it changes yet again. It's a lot of stressing over something you can't do much about. Besides, the max lift loss isn't nearly as important as the area lost from the rocker geometry being out in left field.