75) Why frame-rate mismatch makes subtitles drift
76) A frame-rate conversion re-reads the same span of time under two different clocks. A 100-minute 24fps film has 6000 s × 24 = 144,000 frames; on a 25fps source those 144,000 frames play in 5760 seconds — 96 minutes. The whole film runs 4 minutes short, so a cue at minute 50 appears about 2 minutes late, and the end credits' subtitle never shows up.
77) The formula and the common factors
78) Target rate ÷ original rate = the multiplier. 24 to 25: multiply by 0.96 (24÷25); the other way, 25 to 24: 1.0417. 25 to 23.976: multiply by 0.959; the other way: 1.0427. In the timing editor add a Scale by rate step and put the factor in the value box.
81) When you don't know the two rates
82) If you can't find the source's frame rate, don't guess the factor — use two anchors instead: take the real appearance time of the first and the last cue as two points, and everything between is stretched back proportionally. See subtitles getting more and more out of sync.
83) What to check afterwards
84) Scaling moves every start and end; cue durations stretch by the same factor. After running, verify the two extremes: the first cue (smallest original error) and the last one (largest). If both ends match, everything in between does too.