Yes, that is what to do. But I think you meant to scale the layer/clip first, no? That way you can grab extra area around your content so when looking at the output tab, there will be more room to stretch/warp the mapping points.
But when grabbing exact slices from a large video to be mapped using three projectors with edge blend, it becomes difficult. That was my issue when posting. I believe the answer is to scale down the content inside the layer/composition and overshoot with the projector. Simply because you can't have a slice over another slice. You need room to keep them separated. No?