How much tail material you need to accomplish your checking goal may vary according to the length of the keystick, plus the radius of the arc and placement of the fulcrum. This checking problem came up some time ago, (I believe you raised the question) with some controversy. The conflict arose, (which was not finalized), when you couldn't attain proper checking no matter how much you changed the backcheck angle and/or timing. The issue that didn't get resolved was the role the keystick length played in your checking dilemma. With longer keysticks, the backcheck travels a comparative short distance toward the tail arc, hence giving you greater latitude for checking adjustment. With shorter keysticks, the backcheck travels faster toward the hammer tail. If the tail arc radius and fulcrum are incorrectly calculated, one is forced to backoff the backcheck starting point adjustment to prevent upward checking. That "backing off" now prevents one from obtaining the close checking when the key is fully depressed because the location of an improperly placed arc and arc radius is too far away from the backcheck.
To answer your direct question. If it is truly a Yamaha hammer, you're probably Okay without the need to add more material because your superimposed circle indicates that the fulcrum is properly offset from the hammer shank. I believe it will give you the leeway you're seeking.