The 1-1/2 rule is particularly valid when using bolts up to their full clamping potential. You always want the bolt to fail in tension rather than risk pulling out the female threads. Usually 1 diameter engagement will insure this with a big safety factor.

Rockers are not stressing the studs to that degree (90% proof load) so you could definitely get by with a lot less than 1 1/2 engagement.

For 1/2 - 20 UNF studs 5 turns is 1/4 inch engagement. That represents 50% engagement which should be adequate IMO. After all, the clamp load for a 1/2-20 UNF G8 bolt is 13,000 Lbf.

Kitz