The calculator on iPhones and additional scientific calculator apps can be useful in the conversion of measurements where intersectors and planes are difficult to determine at close distances with very small movements e.g. in a macro shot in a 35mm set up yielding an image of acceptable overall sharpness (not just the peg of extended focus at a fixed aperture), it might be more convenient to then work the formula sine angle of tilt/swing = f/length / distance; if I am using my Canon TS-E 24mm (which I have used and taught for 24 years) and focused on a plane at e.g. 17.5cm from the principal point of the lens, it would be: sine angle of tilt/swing = (f)24/175. Then, a calculator conversion from sine to degrees provides the answer to this input of 8° — the amount of tilt/swing I would be applying for the established angle of the picture. I use a standard Android sci-calc for this; no sweat, no fuss, no confudlements, just cheer.
This calculation is only a good idea at close-ups/macro — it is of no critical consequence in absence in landscape where visual assessment is the most common (and least fiddly) method of establishing the principal planes, hinge line, depth of field, range of acceptable focus and extended focus peg for the given aperature; I imagine in the example I have given that the tilt/swing would effectively be far too great for large format — maybe 35° at an upscaled theoretical comparison to the larger [4x5] format (from 35mm); only very, very small movements of 35mm T/S lenses are required to effect quite large changes and characteristics as opposed to much larger movements in LF.
Unfortunately, while many people with LF aspire to coming to grips with Scheimpflug, in reality it is a very complex, technical and mysterious area of positional theory, not made any more easier by the very deep discourse of Harold Merklinger (albeit with excellent diagrams that clearly reflect what is being explained). So... visual approximation is and will remain the best method of setting measurements/movements — and you should be taking as long as you want until you are happy, the alternative being hopelessly bogged down in calculus and not getting any photography done!