credit score start going down when credit limit usage reach how many %?

You want to stay below about 30% if at all possible, for the best credit score. Your score definitely goes down when you approach 100%.

Carrying balances of 30% of your limit will hurt your score. Pay off the balance and your score immediately rebounds.

Carrying balances is not the same as utilization. It really makes no difference how much of your limit you use, as long as you pay the balance in full every month. It will all even out in the end.

About 30% or so.