Okay so I understand that the higher your activity is the shorter your waiting or post interval would be. But by looking at the code, and substituting my current activity count which is 1106, the waiting time gives me a negative value for >= 15 and >= 60
Should I take the negative as an equivalent for a longer period of waiting time? Or I am just taking the code literally..
Negative means you're allowed to post that many comments at the same time, no more no less

No, simply put the waiting time is calculated for the highest bracket your activity fits into, in this case, >=100 and the result of course is 4 seconds, which is the minim limit unless you get special treatment.