Vertex Manipulation question

I'm trying to make a canopy, supported by a pole at each corner, in Constructor. I have a canvas-looking texture and I've applied it to a 1024x1024 brush which is 2 units thick.

Now I'm trying to subdivide the canvas into 9 squares and use vertex manipulation to move some of the central vertices down - to simulate that the canvas is sagging a bit in the middle.

The middle square, I moved down 4 units. Then the middle square on each side have the inside edge moved down 4 units to match. No problem so far... the problem is when I try to move just one corner of the corner square down to match the others.

I split the corner squares diagonally so that the part that needs to bend down can do so without messing up the other corners, but I can't seem to move the inside corner vertices down properly.

I move the bottom vertex down 4 units and it "jumps" down farther than I intended - to the next grid line. And when I drag the other vertex down, it jumps down to the same grid line and I can't move it back up. Does this explanation make sense?

Is there a better way to accomplish this? or should I use a .dts mesh for this?