My best advice: Eyeball it!
Measurements are an important aspect of what we do and there are lots of calculations for figuring DOF, but some things are best kept in the back of your mind as ‘soft calculations’ while you concentrate on your shot.
Get to know your camera, after awhile you’ll get a feel what each DOF settings will do from trial and error, training and studying how each setting looks. DOF is effected by not only your f-stop, but your focal length and distance.
The DOF preview button that most DSLR’s have is handy, but a pain to use. So I never bother.
Make some days just fun practice – Point, adjust, shoot, look at the results, tweak settings, repeat.
Try different f-stops, bracketing on the same scene without changing anything else, look at the results, what do you notice as different and why. Which ones do you like, and why.
Store that information and your likes in your mental library or notebook of “what happens at these settings” index to use on your next shooting. Then go experiment/practice some more, keeping the DOF theory in mind.
I do mental calculations, guesstimatting hyper-focal distances, knowing that 1/3 back to me and 2/3 beyond my focal point will be in focus, and guesstamate by how far each of those are according to my focal length and f-stop.
Luckily most of my work is landscape while on a tripod, so I know I need f16 or more. Handheld, I’m at ‘happy f8’ if there’s enough light as that’s plenty of DOF most the time, or set my shutter speed to about what my focal length is to reduce movement blur – Concentrating on the scene.
K.I.S.S. Enjoy your day out shooting.