I'll make a suggestion: while the firebox is empty, open the door, and watch the grates while you move the shaker handle back and forth.
Using an average size piece of Nut coal, see how far you can move the grates before the piece of coal could enter the gap between the
grates and jam them open..
THEN use a piece of chalk, or masking tape, or something to mark on the side of the stove the max limits for moving the shaker handle.
This way you can 'see' how far you are moving the grates without seeing the grates.
Once you have the shaking with the short strokes figured out,, you can wipe off the chalk or remove the tape if it hasn't melted off already.
Just a thought..