Determining “Minimum connecting time,” the legal minimum at which an airline will book a connection, is in reality a rather inexact science. And, as most frequent travelers will attest, it is often very unrealistic.

Last weekend, I chose United flights that were a legal connection, at 39 minutes in Chicago, O’Hare, that at other times I might have considered foolhardy, especially since it was a United Express flight landing in terminal 2, transferring to a United plane in terminal 1.

I made the connection, aided by a Air Traffic Control delay on the second flight. Even if it had been on time, I would have made it, albeit during boarding. That may have created a carry-on problem. But in this case, traveling by myself, early in the day, I felt comfortable risking it.

Here are some questions to help determine your own personal minimum connecting time, which may be more than the airlines say.

1. How critical is your connection?
If there are several more flights in the day to the final destination and it’s not during a holiday period, then there’s a decent chance the airline can get you on another later flight.

On the other hand, if you’re connecting to the last flight of the day or an international flight that only goes once a day, make sure to allow extra time.

2. Traveling alone? A single companion may help you run through the airport or flag down a cart; a family will probably slow you down. Besides, a group is as fast as its slowest walker and someone is likely to need a pit stop.

3. Do you have luggage, either checked or significant carry-on? You may be able to run and barely make a connection, but in that case, your luggage may not — and then there’s the whole delayed baggage circus. If you have a large carry-on bag, boarding late may mean a required gate check if the overhead bins are full.

4. Is getting something to eat a priority? With my recent connection, I brought breakfast along, because if the planes were on time, stopping to wait — even in a short line for a sandwich or something — wasn’t going to happen. While airlines generally sell food, it can be limited and may run out.

5. Can you get seats towards the front of the airplane on your first flight? Admittedly, I’m spoiled as a United premier, because I can generally get seats in the Economy Plus section. But, when you’re towards the back of a good-sized plane, it can take a good 10 minutes or so to disembark, especially these days when baggage fees mean that carry-on bins are full and most travelers seem to have the maximum allowed.

6. Do you know the airport? It’s usually much faster when you know what you’re doing and where you’re going.

7. Do you have access to backup? It’s helpful to have a travel agent you can call, a website where you can rebook your travel, etc., in case something goes wrong or you need fast information. (For example, clients often call me upon landing to check their connecting flights and possible backups, before they get to the gate.)

8. What’s your comfort level for a close call? This might be the most important. As stressful as a tight connection can be, airlines don’t generally offer connections their passengers can’t usually make. It’s no fun for them either when they have to deal with unhappy passengers who probably need time-consuming rebooking time, not to mention the luggage problem.

This last question doesn’t have a right or wrong answer. If you’re someone who is relaxed about the concept, figuring that it will work or you’ll work it out, then you probably will be okay, even with a potential close call. If you’re really going to worry about it, before and during the trip, then it’s probably not worth the stress and you should allow yourself extra time.