I'd say +1 as soon as you pass that 5 post minimum to authenticate yourself, then add for time and activity (I'd say +1 year and +1 for every 500 posts), but for rep, is it not possible to make it logarithmic? The linear bonus for rep is what made things spiral out of control. If you got +1 at 10 rep, +2 at 100, +3 at 1,000, etc., you'd see slightly heavier rep sticks as your own rep improved, but not so much.

Would it be possible to provide a small bonus for those who had an award of some sort? +1 for a Silver Compass, +2 for a Gold Compass, +3 for an Atlas Award. Non-cumulative, and with other awards sorted as appropriate. I'd also say that contributing money or time (admin) to the Guild also deserves some extra rep-ability.

For information the member with the most posts is about 11,000. The member with the highest current rep is 85,000. The longest member is about 14 years.
So, if this were one person, that would give +22, +4, and +14. Probably safe to assume at least a Gold Compass at that point and throw in +2. Total: +42. So, in the right ballpark, anyway. If you doubled the rep bonus, that would get to +46. Yeah, that seems better, +2 steps for rep, on the same scale.

Probably could still use some tweaking, but I think that's pretty much what I'd want to see.