Fitting a cue to a fixed duration: how do you budget for the reverb tail?

Suppose a cue must finish within 90 seconds, including its final decay. Would you calculate a tempo adjustment from the entire file, or reserve time for the tail and measure the musical section separately? When would you prefer to revise the ending or render the cue again instead of stretching the existing audio?

Reserve the tail and fit the musical section on its own. At a new tempo the notes move but the reverb decay stays the same length in seconds, so a ratio taken from the whole file comes out wrong.

The sum is tempo = beats x 60 / seconds. Say the cue is 48 bars of 4/4 at 128 BPM, exactly 90 seconds with no room for a tail. Keep 3 seconds for the decay and you need 192 beats in 87 seconds, so 132.4 BPM, about 3.4 percent faster. A plug-in's decay time is the time to fall 60 dB, so reserving that much is safe.

Re-rendered from MIDI, 2 or 3 percent rarely gets noticed. Much past 5 percent the cue starts to sound rushed, and I'd rather cut a bar, shorten the last chord or end on a hit. I'd only stretch rendered audio if the project is gone, and only slightly.

I made a free calculator on my site whose fit-to-duration box does this sum and gives the tempo for a whole number of bars: https://modwheel.net/tools/tempo-calculator