Research guided this.
The judgement was ours.

Shepherd I, Lindley MR, Logan O, Mears AC, Pain MTG, King M · Sports Biomechanics · 24 elite swimmers (12 men, 12 women)

Which parts of a start actually predict a fast time. Trunk position matters, individual limb angles at takeoff do not.

Matúš I, Ružbarský P, Vadašová B · Int. Journal of Environmental Research and Public Health 18(22):11909 · 18 male competitive swimmers, 50 m freestyle 23.3 s

Set-position knee angles and entry angle in competitive swimmers. Our front-knee range started here at 125-140°; the ceiling is now 160°, raised by coaching after every reference swimmer measured above it.

Matúš I, Kandráč R, Vadašová B, Čech P, Ružbarský P · Journal of Physical Education and Sport 20(S6) · 10 swimmers across 15 block configurations, 45 starts each

The only study here that changed swimmers' set-up and timed the result, rather than measuring what swimmers already did. It shaped how we think about back-knee bend; the range we hold you to is the coaching table's, not this paper's.

Matúš I, Němá K, Eliaš T · Sport Mont 24(20):179–194 · 18 studies of the racing start, screened from 665

What the published literature says about time in the air. We measure the flight phase the way these studies do, from the block to the first touch of the water, so we show you their figures. We hold you to no target: across the 18 studies the phase spans 0.27 to 0.41 s, wider than any one swimmer's own reading is steady to.

van Dijk MP, Beek PJ, van Soest AJK · PLoS ONE 15(10):e0241345 · 14 sub-elite and elite swimmers, 349 starts

Entry angle across 349 starts, backing up the range above independently. Its 38 ± 7 is where the 32–44° band we hold you to comes from. A tighter ceiling would fault swimmers for entering the way this population does. We apply that band to your hands and your legs separately (Hands-in angle, Feet-in angle) rather than to the whole-body line, which can sit inside the range while both halves are wrong in opposite directions.

Slawson SE, Chakravorti N, Conway PP, Cossor J, West AA · Procedia Engineering 34:801–806 · 10 national-level sprint swimmers on a force-instrumented block

Set-position knee angles in the fastest starts, measured against actual force production.

Rudnik DM, Rejman M, Vilas-Boas JP · Frontiers in Physiology 14:1157359 · 52 international swimmers (30 women, 22 men), 750+ World Aquatics points

The flight-time range in the Elite column, and the only study here that measured women and men separately. It reads the entry angle the way this app does, fingertips to hip against the horizontal at the moment the hands go in, and its 37–39° is the third population to agree on that figure. It is also the one study that times the block phase from the swimmer's first movement rather than from the starting signal, which is the window this app measures.

Tor E · PhD thesis, Victoria University · 14 elite swimmers on an instrumented OSB11 block

Where block time and entry distance sit for elite swimmers, and the difference between the angle of your body and the angle you are actually travelling. That is why this app measures both.

Tor E, Pease DL, Ball KA · Journal of Sports Sciences 33(12):1313–1321 · 52 starts by elite freestyle and butterfly swimmers

Which part of the dive is worth measuring at all. Of everything that happens above the water, 81% of what separates a fast start from a slow one is how quickly you are travelling forwards when you leave the block, not the shape you make in the air. It is also honest about the limit of this app: the underwater phase, which we cannot film, decides more than the part we can.

Born D-P, Nussbaumer L, Buck M, Ruiz-Navarro JJ, Romann M · Bioengineering 13(2):180 · 136 national-team swimmers, including Olympic medallists and a world-record holder

The take-off speed comparison range. Not a target. Even its slowest swimmers are national-team level, so it shows where a number sits rather than grading it.