The % change in tempo works smooth but it might work different from what you're thinking. Let me give you an example.

Lets say current tempo at bar 12 is 100 BPM. You want bar 13 to REALLY slow down so in bar 13 F5 bar settings you enter a % change in tempo value of -50. Minus 50 tells BiaB to slow bar 13 by fifty percent or half the tempo which is 50 BPM. Now in bar 14 you want the tempo to slow down to about 40 BPM, what % change in tempo value do you enter? Did you think 10 because 50 - 40 = 10? Nope. The answer is -20 because you want bar 14 to slow down to 40 BPM. The minus sign tells BiaB to slowdown and the 20 is 20 percent of 50 which equals 10 and 50 BPM minus 10 BPM equals the 40 BPM we want in bar 14.

Now you know why many people say music is math.

Jim Fogle - 2025 BiaB (Build 1128) RB (Build 5) - Ultra+ PAK
DAWs: Cakewalk Sonar - Standalone: Zoom MRS-8
Laptop: i3 Win 10, 8GB ram 500GB HDD
Desktop: i7 Win 11, 12GB ram 256GB SSD, 4 TB HDD
Music at: https://fogle622.wix.com/fogle622-audio-home