In AE, Mocha, how to get mask to track evenly when the corner points don't exist ?

Still working this one... hours of frustration.

In Mocha, trying to mask the bus ad so I can replace it.  It moves from left to right.

I'm replacing the bench ad with a new bench ad.

Here is how it starts:

I made an estimate of where the left side would be.

But when I track it,  the left and right side goes out of whack and I get this:

If I manually adjust the splines, they never come out perfectly square, because I have to "guess" where the bench ad will be, because it goes out of frame.

Has to be a perfect mask, because I'm replacing the image, and when the mask goes crooked, the new ad image bends and warps.

I tried the "AdjustTrack"  and it gave me the 4 red points, but how can I install the 4 points, when 2 of my starting points are out of the frame?  I tried, didn't work.

The tutorials all have masks that are entirely in the frame. 

My left side starts out of frame, and the right side later goes out of frame.

I've seen 6 videos on AdjustTrack in Mocha and I can't find one with my problem.

Thanks for your help and time! or a tutorial referral would be great.

Letty