13 to 14 hours since last dose is too long. That could account for the low level. As Christina said, it has to be within 10-12 hours. They should ask you when you have your blood drawn. Can your doctor order another test?

My pdoc says .6 is the lowest therapeutic level (for me, anyway, not sure if they go by ht/wt). I take 900 mg and will probably stay at that since my levels are creeping up, but still no where near toxic levels.