I prefer Fixe SS ring anchors. As Mark mentioned, 6-8 inches apart horizontally. Any further and ropes will have a tendency to twist. I also use construction grade 3/8 x 3 1/2" SS bolts. Embedment does effect strength despite rock type, a bolt embeded in 2" of rock is twice as strong as a bolt embeded in 1 1/2" of rock. Non stainless bolts can fail sooner than you'd think mostly due to water contact with parts of the bolt we can't see-- it doesn't take much. Those old non stainless 1/4" bolts you guys used to use have now tested to have failing strengths within 1 year of placement.
You can buy SS bolts at confast.com for minimal cost. $78 per box of 50 for the Chinese variety online, or call and order American made bolts (from American materials) for $2 a pop, which make for a great set-up with the Fixe hardware.
If you're ordering large quantities of Fixe hardware, make a phone call to Fixe.