First you need to see where you are to start with using the 1.8 rockers and a stock length pushrod. Also, just watch the contact pattern on the valve tip as you are rotating the engine to see if you are even in the ballpark with the stock pushrod. Make a mental note of where the rocker tip is at the start, midlift and full lift points on the valve tip making sure it stays on the valve tip. Changing to a bigger ratio rocker will likely make the engine need a longer pushrod, if everything was correct to begin with before the change. If you have a checking pushrod also, you can shorten or lengthen it to see how it changes the rocker tip contact area on the valve tip. The optimum is as has been mentioned, where the rocker tip is in a close proximity of the valve tip center at mid lift. But first, you will have to see where the rocker wants to begin, either at the inside or outside of the valve tip and go from there.
