Beta hCG expectations after low rise.

My hcg was 67 at 12dpo, 137 at 14dpo, and 391 at 18dpo, 96hrs after last draw. That last rise was a 69% with a double time of 63.5 hours. Today is 20dpo. Given my last rise, what should my hcg be today, and at what percentage rise should i prepare for a miscarriage?