I don't know if I can explain it well, but I think the Timing Tool isn't designed to allow users to specify a desired stretch percentage. The percentage that temporarily appears in the right corner of the clip is just a
resulted percentage calculated from the number of bars, ticks, etc. the user dragged. This means that even if the tool allowed users to time-stretch multiple clips at once, clips of different lengths will only be slip-stretched by the same number of bars, ticks, etc., not by the same percentage.
(As for the practicality, I find the tool practical because I prefer to stretch multiple clips one by one when they are located far away from one another.)
So, as a workaround, I suggest you display all MIDI tracks in the PRV, select the notes you want to stretch, then hold down Ctrl and drag the right edge of the selection to left/right, as shown in the video below that I just captured. Sorry if you already knew this feature.