For those like me used to BG meters that measure in mmol/l (I have a diabetic partner) the rule is to divide by 18:

• 120 mg/dL ≈ 6.7 mmol/l
• 160 mg/dL ≈ 8.9 mmol/l
• 200 mg/dL ≈ 11.1 mmol/l
• 250 mg/dL ≈ 13.9 mmol/l

since glucose is about 180 daltons, but then there's a factor of ten in the conversion from decilitres to litres