extrapolation

I am into purchasing. I am fond of doing my self developed formula on extrapolation and forecasting.

If my bchg is 99 on day1, then if it triples on day 3. I guess it will triple again on day 5. So far, my predictions were correct. But now, my forecast deviates from the actual value. 

I am afraid to analyse my bhcg values. I hope 23893 is a good number.