Tracking BBT — how long before it actually tells you anything useful?

Started basal body temperature tracking this cycle. Three weeks in and my chart looks like a stock market crash — all over the place. My gynae said it takes a few cycles to see a pattern. For those who used BBT tracking: how long before you could reliably spot ovulation? Any tips for making it more accurate? I'm using a regular thermometer but wondering if a dedicated BBT thermometer makes a difference.