Most normal people are running between 6 and 8 over in the surf but would be worth starting at 8 and taping it and try it for a week before you cut it down any more. also consider what other boards you my be riding in the surf or are you planning a purchase in the near future because what works for the board you are on now may be too short if you suddenly jump on a board with heaps more float. So I guess work out what feel good in the surf and take into account the biggest board you will be surfing.
Mine is 6 over tend to hit the board less when it gets ugly :-)
No rules some go 12 over some same as their height. Tape up cut a little at a time and don't be in a hurry give it a few sessions before you cut again. If you know someone with one cut already might be worth swapping over in the surf that is how I came up with 6 over for me.
i am about 3"over for surf and about to go 9"for distance,annie is about the same for distance and head high for surf,however the last cut to head high gave the paddle no flex so be wary if your a shorty about paddle flex