One division of the Dashamsha spans exactly 3 degrees, ten to a sign and 120 in the zodiac. The ascendant crosses three degrees in about twelve minutes, so a birth time uncertain by a quarter of an hour puts the rising dashamsha in doubt.
The counting rule is settled: in an odd sign the ten divisions are counted from that sign onward, and in an even sign from the ninth sign from it.
The D-10 does not appear in the shadvarga or the saptavarga. It enters at the dashavarga of ten and continues in the shodashavarga, which places it in the second rank of weighted charts: consulted routinely, but not part of the compact six that a classical strength estimate begins from.
That position suits its subject. Career, standing and public achievement are late-arriving departments of a life, and jyotisha treats them as a specialisation of the tenth house rather than as a general index of a nativity. An astrologer reading the D-10 is answering a question about work; the same astrologer estimating whether a planet is strong at all goes back to the D-1 and the D-9.
Because the division is a whole number of degrees, the D-10 has none of the rounding trouble that afflicts the D-7, and every boundary falls on a round figure that two programs will agree about.