Unless your base dominion is high, the chance to get +1 candle with one slave is very small. For exemple if you've a max_dom of 6, the chance is probably 15%. Curiously, I've sometime seen a new candle in an adjacent province and not in the sacrifice province [img]/threads/images/Graemlins/Cold.gif[/img]! So it's very hard to compute. If you want an higher %, build more temples... you get +1 max_dom for every 5 temples.
And remember that your pretender gets a free temple check every turn, and this one is a "true", standard check. Personaly I dont sacrifice slaves in the beginning of the game, the pretender alone is enough to reach 4-5 candles in the capitol.
Cheers