The problem with any formula -- such as sticking with one particular brand of tires -- is that there are too many variables. We won't know for sure until we try the tire on a rim, or get reliable measurements from someone else who already has the same set of tires and rims.

Continental tires don't all run wide. Conti Speed Rides are nominally 700x42 but everyone who rides 'em agrees they're actually closer to 700x38, which is what they measure on my rims.

Conti Ultra Sport II are very tight fitting but actual size is nearly dead on to the specs. Ditto Schwalbe One.

Michelin Protek tires, especially their Cross and Max versions, generally run quite a bit wider than spec. The 700x40 nominal size for Protek Cross Max is well short of the actual measured 700x45 width. No idea about their road tires, haven't tried 'em.

And if a manufacturer switches factories or makes any change in a favorite tire -- such as when Schwalbe revamped the Pro V-Guard into the Pro One tubeless -- all bets are off.