Taxi slang: Making one's knot

Factual Questions
1

Just read a book that claimed that a taxi driver’s daily expenses are sometimes called his “knot,” and that once a driver takes in enough fares in a day to cover his expenses, he has “made (or earned) his knot.” Everything after that is profit.

I’ve never heard this phrase before and it doesn’t show up on any web searches.

In gambling, a gambler’s expenses are referred to as his “nut,” and one who earns enough gambling to make a living at is has said to have made or cracked the nut. This is slightly different, in that one’s “nut” is all of one’s expenses, like food, rent, etc. The taxi driver’s “knot,” as far as I could tell, was just the cab related expenses–one could make one’s knot and still be unable to make ends meet.

What’s the straight dope? Is this really a phrase, or did the author mishear knot for nut? Any relationship between the two phrases? And finally, any other professions that have unusual terms for success (e.g., making the “show” in baseball)?

2

Reference here:

“To break even. A taxi driver is said to have made the knot when (s)he has made enough money in fares to pay the daily car lease.”

3

“Nut” has long been used to mean business-related expenses (see here, #7. It is not just a gambling term, and is not limited to a specific grouping of expenses (e.g. only daily expenses vs. everything vs. start-up costs).

It seems clear to me that someone somewhere misheard “nut” as “knot,” and it has spread slightly. Emphasis on slightly, because I’m sure you’ll find a jillion incidents of using “nut” for each incident of using “knot” for this meaning.