Maintain spacing between two text layers

Maybe this will get you started. This will position any text layer 90 pixels to the right of any other text layer below it and still allow you to animate the Y position.  You could replace the pad with an expression control slider.

lyrOfst = thisLayer.sourceRectAtTime().left;
pad = 90;
mstr = thisComp.layer(index + 1);
mstrSize = mstr.sourceRectAtTime();
mstrWidth = mstrSize.width;
mstrLeft = mstrSize.left;
mstrPosX = mstr.position[0];
x = mstrPosX + mstrLeft + mstrWidth - lyrOfst + pad;
y = mstr.position[1] - (thisComp.height/2) + position[1];
[x, y]

With a little work, you could even get the entire move automated using the in points of the layers. I have about 50 Animation

Presets that work like that. Then animate a layer in and out based on the layer in and out points. 

Here's how sourceRectAtTime() works.

sourceRectAtTime().width retiurns the width of a text layer or a shape layer not including the stroke width

sourceRectAtTime().height returns the height of a text layer or a shape layer not including the stroke width

sourceRectAtTime().top returns the distance from the baseline of a text layer

sourceRectAtTime().left returns the distance from tha anchor point of the text layer

You have to use a combination of all of these to compensate for any baseline shift or changes to paragraph justification. The text layer anchor point must be at 0, 0 for source rect at time to accurately adjust the position of other layers. 

I don't have time to work out the entire expression for you now but it will require references to the text layer position and all of the sourceRectAtTime() properties to set proper and consistent spacing for your project.

The usual sourceRectAtTime() expressions will work just fine. A simple web search will turn up plenty of examples, as it was everybody's favorite new feature two years ago and they felt they needed to do a tutorial on it. The rest is just basic math - width (of sourceRect A) plus position A is the position of sourceRect B and "outside the comp" is simply sourceRect B's width subtracted from the left edge (zero origin). The only real question is the time at which the rectangles get sampled to not interfere with your keyframed animation, but it seems you would simply use the time when the A segment has settled, so this would be key(3).time.

Mylenium