The tolerance is not centred on zero.

Audio arriving early is far more objectionable than audio arriving late, and the reason is physical rather than arbitrary: in the world, sound always lags light. A door slams across a courtyard and the bang arrives after the picture of it, every time. The ear has a lifetime of experience of that and none at all of the reverse. So the window is lopsided — 45 ms of lead against 125 ms of lag, nearly three times as much room in one direction as the other.

The engine is free and needs no account. Which means a report that says “sync error: 60 ms” has thrown away the half of the measurement that decides whether it matters: 60 late is comfortably inside the window and 60 early is outside it. Write the sign and the page judges every offset against the limit for its own direction, says what the same number would have meant reversed, works out what a whole-frame move can actually achieve — half a frame at 25 is 20 ms, nearly half the entire early budget — and tells a slip from a slope, because a constant offset and a drift look identical at one measurement and need completely different fixes. Every lane ships a worked example that costs nothing. Writing the report needs credits.