dilletant
The only way to scale all tempo events proportionally is "Fit To Time", but it's trial-and-error. There is no percentage or "old BPM - new BPM" option.

There is, but you need to calculate total frames:
 
1. Enable Auto-stretch (a.k.a. Clip Follows Project in Auto-stretch mode) on all audio clips.     
2. Select a MIDI or audio clip that runs the length of the project (or select in the timeline).     
3. Go to Process > Fit to Time.     
4. Convert the current Thru time to total frames (usually 30 frames/sec).     
5. Divide that by the desired tempo change factor (e.g. 1.10 to get a 10% increase, 0.90 for a 10% decease).     
6. Round and convert that new value back to minutes:seconds:frames.  
7. Enter that as the new Thru, and select Modify by Changing: Tempo Map.     
8. Click OK.