Your method of choosing initial saddle height is correct, with this addition: pedal a few strokes using your heels, and make sure your hips are not rocking, i.e. you are not stretching to reach the pedal with your heel.

That gets you in the ballpark. You might then tweak saddle height depending on your preferences. Some people pedal toes down, others heel down. If you spin a high cadence you might prefer a slightly lower saddle than if you mash a low cadence. And some just prefer a more or less extended leg.

Don't rely on the bike shop to adjust your bike's fit for you. Some are better at it, others don't even try, a few just want to sell the bike in stock. Bikes are adjustable with just a couple Allen wrenches. Do it yourself.

Of course, you need to buy a bike that is generally the right size given your body dimensions, close enough to be adjusted to you. Use a fit calculator (example here Bike Fit Calculator | Find Your Bike Size | Competitive Cyclist ).