Ideally, if you are on axis and adjust the zoom lens only then theoretically the blend will overlap the same each time.

Let's imagine your projectors are 3 meters apart. Each image will be 4 meters wide. The total image will be 7 meters wide with a 25% blend.

If the axis of each projector is parallel on a horizontal plane then as long as you zoom the projectors to be 4 meters wide, your total image will always be 7 meters wide and your blend will always be 25%.

An issue you haven't considered is the blend curve in catalyst. Yes the blend % will always be the same but you will have to change the blend curve each time the distance changes. This is because the feathering of the edges is inversely proportionate to angle of the projection.

Basically, if the projectors are further from the screen (long throw) then the blend curve will be closer to linear. If the projectors are closer however, the blend curve becomes much more quadratic.

Don't worry though. You'll see what I mean and honestly, it's not a complex change.