You first part is basically correct. Most servo's have about 45 degrees of travel both directions. So if you want 45 degrees of throw, the hole on the servo arm needs to be at the same radius as the hole on the control surface with respect to hinge line. If you want less throw, then the servo hole radius needs to be less than the control radius.

You have the err direction correct. It is better to err on with the servo arm being shorter. If you go the other way (long servo arm, short control horn) you decrease servo authority and can induce flutter. But, don't get too short as that can induce flutter. I.E., You are better off using two long arms as opposed to two short arms.

As for binding, yes you need to be sure the linkage won't hit airframe, etc.

As others posted, I wouldn't go 140 on the ATV unless you have to. I believe it is OK to overdrive some, say to 120. If you overdrive too much, there isn't enough room for any mix headroom. Also, some servos stop moving if overdriven, i.e., they hit their end of movement say at 130, etc.

So, once you have your ATV set (say at 120), try to get your throw set mechanically without messing with ATV. This is almost obtainable with adjustable horns (like threaded horns), and messing with the servo horn orientation on the servo splines. If you use the fixed hole horns, just get it as close as you can. Then tweak with ATV. If you were close with the mechanical side, you shouldn't need to adjust your ATV by more than about 5 either direction.

Now your ATV is set. There is no need to ever mess with it again. For low rates, you just set the rate which is a percentage of the ATV you set. So if you want 10 degrees, and your ATV is set to 45, your D/R setting would be around 25%. But since servo's aren't linear in their movement, be sure to measure the movement. Up/Down DR may not be the same for 10 degrees. This is due to linkage issues and servo pot issues. So as you put it, you are dialing the D/R down and the ATV stays up.

This does reduce resolution, but as others posted, there is no way to avoid this on any radio. That is the penalty of having extreme throws.

Oh, and ditto on the expo. On Futaba, you'll want negative expo, JR is positive. If your radio supports different expo on high vs low rate, use it. Otherwise I'd set expo on elevator to something around -40 to -50 which should work OK in both high and low rates, but different expo in high vs low rate would be better.

Cheers.