Thanks for this information about tempo.
I notice that when you press f5 to change tempo it doesn't have a "return to normal" option (the way it has for volume).

And if you decrease it by -10, then later increase it by plus 10, it doesn't return to the original volume. I suppose you would have to increase it by 10/9 , or 11.111... to get it back to the original volume.

So does this mean I am better to put in values for the tempo instead of percent changed? I have entered "Old Man River", and it seems slow down and then resume speed repeatedly. I'd rather use the percentages so I could change the overall tempo more easily.