I think you're right, TGZ: With a gesture, pointing to what level on the coffee cup I want the hot water added, "
ใส่นิดเดียว" and maybe adding "
ถึงนี้" or "
ประมาณนี้" could work.
The problem is that in practice, since the coffee is delivered in a cup that's about 3/4 full, the barista often misses the mark and pours too much water in. They don't pour just a small spritz at a time and then look to me for confirmation. It's all in one fell swoop. I've offered to pour the water in myself. It can be done but there are disadvantages to that approach.
I vetted the use of "
เหยาะ" with a native Thai speaker and came up with this:
"
เหยาะ" as a noun: "
น้องๆ กาแฟมันเข้มไปหน่อย รบกวนใส่น้ำร้อนสักเหยาะนะ" Using
เหยาะ as a verb "
รบกวนเหยาะน้ำร้อนอีกหน่อยหนึ่ง" This is
ภาษาพูด of course so it's doesn't abide any strict usage rules.
Haven't tried it in the field yet, but we'll see! I'm interested to see if I use "
เหยาะ" will the barista use a container that's more appropriate for
เหยาะ'ing than for pouring. Or, will she just pour using the same kettle-thing in much more graduated way until I say "enough"...Or neither
What I like about instructing the barista to "
เหยาะ" is that the hot water is gradually added in a series of smaller quantities, with a brief pause between each "
เหยาะ". During that series of
เหยาะ's I can watch how high the water is getting and can say "That's enough" at any time at the precise level that I want. Almost no risk of the sudden deluge and overfill that I have encountered with
ใส่น้ำร้อนนิดเดียว - which is just too open ended/ambiguous.
Also, I like the idea of using
เหยาะ because it "informs" the listener about me, the speaker - my mood, my personality, how adept at the language I might be and other subtle things. I sometimes like to move beyond the more literal expression and
เล่นคำบ้านๆ a bit...using the clever sounding, pithy, funky expressions that the Thai native speakers use. It shows that I know the "local color", and can portray me (or any speaker) as - cool, funny, clever, dialed-in etc. (depending on which expression I am using).