bottom line: you need (at least) 2 exact breakpoints.
method 1 (drag points against visible range)
set the visible range of the tempo envelope temporarily to the target value. (these are the two boxes to the right of the envelope pane, in the master track) then you can drag a breakpoint (not a segment unfortunately!) to the limit and it will hit that exact value. later you can resize the envelope display to whatever..
or, you can get started with a segment drag, and finish up by dragging the points:
-make a time selection directly in the envelope field, and drag a segment. that creates four breakpoints.
-drag the two outer points to the limit. they will be constrained horizontally by the other two points, if you drag against them (and aren't pressing shift!)
method 2 (punch in/out)
NB there must be at least one breakpoint on the envelope before doing this (so that the envelope greys out and the "back to arrange" light goes on when you enter your new value):
-make a time selection: grid, no grid, or clip-based
-right-click: loop selection
-type the tempo into the box up top
-punch in, then out
now you have the first envelope point (and an extra 2 which you will delete)
-put the cursor near the end of the loop region
-enter the tempo again (or another tempo, if you want to make a perfect ramp, but in this case switch off loop and switch on punch-out, so that you don't mess up the beginning of the envelope)
-punch in and let the loop come around, punch out
now you have the last envelope point
-delete the 4 extra points in the middle. (tip: you can use shift+drag to pull on one of the extra points and delete the other 3, then you only have to delete one point)
if the time selection is short enough that you can wait through it, you can just let the envelope record
method 2.5 (punch in/out, with midi or key assignment to tempo)
-use a midi or key assignment with a range that gives predictable numbers. this is most helpful if you want round numbers for the tempo. it's quicker to set tempos this way than typing into the field. e.g. a key range c2..c4 with range 100..124 will give 1bpm increments over that range. midi assignments will be even if the range is an even mult/div of 128. (i have sometimes seen errors this way, increments vs. decrements. i.e. it will go up in even numbers, but on the way back down will be off by some decimal place. ??)
once your assignment is set up, you can do your envelope punches more efficiently
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