Hey Skyhawk,
Unfortunately a tipster can never be 100% Correct, if they where, bookmakers would be out of business. I is generally all about the horses form, the particular jockey, the draw they are give, and of course the race conditions on the day.
I highly recommend Matthew Stevens, and would also recommend you look at africanbettingclan.co.za as this is a forum dedicated to South African Horse Racing, and will generally give you some good insight.
If you need any further information, please feel free to PM me.