The day formula stored for this lot is Ascendant plus Mercury minus the Lot of Fortune. Fortune by day is Ascendant plus Moon minus Sun, and substituting gives Ascendant plus Mercury minus Ascendant minus Moon plus Sun. The Ascendant cancels, leaving Mercury plus Sun minus Moon.
By day the Lot of Necessity contains no angle. It is built from three planetary longitudes, so it is indifferent to the place of birth and nearly indifferent to the hour.
How nearly depends mostly on the Moon, which is subtracted here and is by far the fastest term. The Moon covers about 13 degrees 11 minutes a day and the Sun about 59 minutes, while Mercury varies from about two degrees a day down through stationary to retrograde. The three together move the lot roughly eleven degrees a day in the ordinary case, which is about one degree every two and a quarter hours. A birth time wrong by two hours moves this lot by about a degree; the same error moves the Ascendant by thirty.
It is worth pausing on how counter-intuitive that is against the formula as written, which begins with the Ascendant and looks as time-critical as any other lot on the page. It is not, and an astrologer working from a rough birth time can use Necessity in a day chart with more confidence than almost anything else in the horoscope.
As with the other lots derived from Fortune, this reduction is specific to a day birth and must be redone for a night one.