Elad pointed out a question that the challal of the mem is a problem. Indeed I think that if the majority of the challal of a mem or samech are blocked by ink, they are pasul as a shinuy tzura.
We find in Alfa Beisa that lechatchila the challal of the beis should be wide [hight] 1k, but doesn’t explain bdieved the minimum a challal may be. The Maharsham (vol. 7:112, 115) asked on a caf that its challal was very thin as a crack, he paskened that its kosher, since no shiyur in the letters challal are m'akev bdieved. I understand that the Maharsham would machshir also if the majority of the length [right-left] also was blocked [illustration 1], as long as this doesn’t constitute a shinuy zura.
But, see yalkut hasofer p. 65 pic. 127, in name of Me'asef l'chol Hamachanos that the caf that its challal was thin as crack is pasul.
I would think that the mem must have a full kulmus length of challal horizontal, less than 1k I doubt its kosher [illustration 3]. Also if it has a 1kulmus challal vertical alone [illustration 2] I am in doubt if this is kosher. [In Osiyos Harav p. 230 par. dalet, I wrote that they are pasul].
I am also doubtful if a hai or tav [illustration 4. The illustrations of the hai & tav are wider than what I actualy mean!] are very thin, are they kosher, because they are lacking a correct challal. [Biur Hasofer p. 47, and yalkut pic. 61 writes that the hai is kosher, if the gag has 1k length. He doesn’t consider a problem at all in the measure of the challal].
But I have a proof that there is hallachicaly [l'ikuva] no measure in the challal of the letters, as the Maharsham wrote, from the psak of the Tur & Shulchan Aruch 32:15 if the letters hai or mem were preforated [after they were written properly, nikav afilu col challalam, see SA Harav there] and no klaf was left inside the letters, they are kosher [the minor opinion arguing, is only because the law of hekef gvil].
Now if the challal was a vital entity of the letter it would need a minimal shiyur, as the foot of the hai that was cut has a minimum shiyur - MOK. From this I conclude thare is no shiyur for the challal and even if lost completely doesn’t pasel the letter, obviously as long as the letter is recognizable.
So the conclusion should be l'hallacha, that a letter that has a minimal challal or its challal has been blocked partialy by ink, is dealt as a question on shinuy zura only.