One division of the Vimshamsha spans 1 degree 30 minutes, twenty to a sign and 240 in the zodiac. The ascendant crosses that arc in about six minutes.
Six minutes is less than the error in most birth times that were not written down at the time. The practical rule that follows is not that the D-20 is unusable but that its ascendant and its house structure are unusable without a recorded time, while the planetary placements in it remain sound. The Sun moves less than a degree a day and the outer planets far less, so their vimshamsha divisions are stable across a whole day of uncertainty. The Moon is the exception among the grahas, crossing 1 degree 30 minutes in roughly two and three quarter hours.
That split, stable planets and an unstable ascendant, is general to every varga above the D-12, and it is the single most useful thing to know when working from an approximate time. Read the planets, and say that the houses are not available.
The Vimshamsha is one of the six charts that enter only at the full shodashavarga. Its subject is upasana, spiritual practice and devotion, the deity a person turns to and the manner of their turning, which the tradition treats as a narrow question rather than an index of general strength.