Efficient way of handling percentile chance of event in programmatic way

I am developing a small app like game, I have done this before and still not sure if it was the correct way to do it. What I need is percentile chance of some event, for example gaining an item after win 10% chance.

I have been using random number generator each time on server side, if number is <= 10 then user will gain reward, but it still does not satisfy 10% criteria across the all users environment.

I was thinking about recording user's turn number on server side and reward an item every nth time, but don't know if it's right way to do it. I would like to know about your ideas of doing it and suggestions. Also I did not know if stackoverflow is right place to post this or any other community of stackexchange group. If so please guide me in comment and I'll move question in appropriate community. Thanks.

via Ankit Pise