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The expression is actually "down to a tee" or, more commonly simply "to a tee". Either "tee" or "T" will do, but in any case the reference is to the LETTER of the alphabet.

This is clear when you discover the origin of the expression. It's actually a shortened form of "to a tittle", an expression in use in English by the early 17th century, with the meaning "to the smallest detail." (the variation appears by the late 17th century)

The word "tittle" comes from the Latin word for a diacritical mark (and is related to the word "title").

http://www.thefreedictionary.com/tittle

But the key to the meaning of all these expressions was John Wycliffe 14th century English translation of the Latin Bible. In Matthew 5:18, where the Latin has the word "apex" (the original word in the Greek literally means "horn"). Wycliffe chose the word "tittle", thus referring to a tiny pen mark that distinguished a letter.

It was a good choice. In this verse Jesus refers to very small marks on the top of certain Hebrew letters that distinguished them from very similar Hebrew letters, rather like the small stroke that distinguishes our capital Q from a capital O or G from C. (Some modern translations use expressions like "least stroke of a pen" to convey the idea.)

http://www.randomhouse.com/wotd/index.pperl?date=1...

Note that "tittle" in this verse is is the second member of a pair, the now familiar "jot and tittle". The term "jot" renders Greek "iota" -- the name of the small Greek letter "i", though Jesus probably was speaking of the tiny equivalent HEBREW letter "yod".

http://www.randomhouse.com/wotd/index.pperl?date=2...

"Jot" and "tittle" continued to be used in later the 16th century English Bible translations (beginning William Tyndale in the 1520s). Many still know these terms from the King James (1611) translation -- "Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled."

http://www.blueletterbible.org/kjv/Mat/Mat005.html...

So, based on this Biblical "tittle" to refer to tiny details people began to use the expression "to a tittle" --and other later variations of it-- to refer to something done very precisely (that is 'to the smallest detail').

see also:

http://worldwidewords.org/qa/qa-toa2.htm