You can start there. Then you listen to the mic as you move the mic away from the center, towards the outer edge of the speaker. Depending on about a guzillion factors ( speaker size and type, mic,cabinet, amp, player, guitar, room, time of day,etc), you will eventually find the "sweet spot". I tend to NOT point the 57 ( or 421 or 635) directly at the center because it's just to bright and harsh there. I usually decide on a point that's closer to the outer edge, but you need to experiment to find what works for you...

I start halfway between the dust cover and surround right up on the grille if I am going for a strong rhythm tone. I actually move about 2 inches back for leads. Then I start trying to find where I want it to be.