Prior to eating I count carbs and then use my insulin carb ratios to determine how much insulin I should take. This is straight forward when I'm cooking my food, going to a chain restaurant, or have clear carb numbers.
Sometimes however I end up in a restaurant where no carb information is available. I order my food and then guess how many carbs there are. I try to err on the side of being too low so that if anything my blood sugar will go high and I can take a correction dose.
What I do today is something like this.
3:00pm - Took 5 units of insulin for Dinner #3 at X restaurant. Pre meal BGL was 85. 5:00pm - BGL is 200
Then I say my BGL went up 115, next time take more insulin. Next time I will add a couple of units. I keep doing this until I get 'close'. What would be ideal is if there is some formula I can use to calculate how many carbs that was, so the next time I eat the same thing I can dose correctly.