This kind of thread is why I joined this forum!!
My solution might not work for everyone, but since I make filter coffee at home, I have a scale on the table that I use every day. So I got in the habit of placing my cup on the scale whether I’m making coffee (where the scale is needed) or tea (not needed). My mug holds about 370 ml, so I pour 300 ml of water, then add milk if needed. I always do 300 ml whether it’s coffee or tea.
So perhaps get a bigger mug and pour a bit less…? (“That’s only gonna make me wanna fill up this bigger mug” is a valid response, I’m afraid)