I think it averages around 1cm for every week, so for example 30cm at 30 weeks. So sounds like you are pretty much spot on.
Measuring externally isn't a very effective measurement anyway as a lot can make a difference, such as bloating or position of the baby. If suddenly you were measuring way differently or there wasn't much growth (or too much) then you would be sent for a scan which is a more accurate way of measuring the baby.