“Down to a T” is the correct expression in English. “To a T” means something is exactly right.
The expression is common in English when someone is amazed that something is so exact.
Oh wow, you got the design down to a T. The dress looks exactly like the one I used to have.
“Down to a T” is a set expression, you need to use “a” and a capital “T”.
“Down to a T” is usually used when there is some sort of measurement.
He is a great carpenter, His measurements are always to a T.
(Down) To a T
The word “Down” is optional in the expression and usually adds more emphasis and amazement.
Down to the T, Down to a Tee
“Down to the T” and “Down to a Tee” are common misspellings of the correct form “Down to a T”
What does follow to the T mean?
“Follow to the T” means to follow something exactly.