For a while I have been not sure about the correct way to write these characters and their variants. It seems both in hand-writing and in fonts, the position the third stroke starts varies.

Is anyone able to clarify what the correct range of starting positions for the third stroke are?

(I have tried googling for it without much luck).

巳 [sì] which is the the sixth of the twelve Earthly Branches is completely closed.
The 己 of 自己 is completely open.
The 已 of 已經 is halfway open.

Try using this "開口己，半口已，閉口巳" 順口溜 to remember them.

Thanks! That is very helpful!