If its a nice new pack, 10-20a should be perfectly fine. I have batteries I've charged at 20a most of their life with little degredation in their performance. Above 20 and up to 40 gives you a temporary boost in performance but hurts their life.

But I have 4 reedy 6100s that are right at a year old and their IR's are 1.6-1.8 per cell now and they were 1.0-1.1 last year when new. They've mostly been charged at 40a but lately a lot has been 20. So like I said earlier 10-20a is perfectly safe, still quick and not really damaging.

Also important is good charge leads. I've tried a few and not been super impressed but Im going to try some Reedy ones next. A way to tell is if you move the cord in the battery or charger and it changes the charge voltage. Keep that in mind and charging will be plenty safe.