The Shashtiamsa is the finest of the sixteen vargas of Brihat Parashara Hora Shastra ch. 6: each sign of 30 degrees is cut into sixty parts of 30 arcminutes, counted forward from the sign itself, and each part carries a named amsa deity. BPHS assigns it the greatest weight of the divisions, and it is read for karma and for the totality of the life.
The construction is the simplest of all the vargas: sixty equal parts of 30 arcminutes, counted forward from the occupied sign, with no modality or parity rule to remember. What makes the D60 hard is not the arithmetic but the precision it demands of its input.
Each part also carries a name. The sixty amsa deities run in a fixed order from Ghora and Rakshasa at the start to Bhramana and Chandrarekha at the end, and in an even sign the same list is read in reverse. Phaladeepika ch. 6 marks twenty-four of the sixty as krura, that is malefic, and the rest as saumya, so in this division the name of the part carries a quality that the sign alone does not.
A shashtiamsa is 30 arcminutes wide. The ascendant moves about a degree every four minutes, so it crosses a full D60 part in roughly two minutes of clock time. A birth time recorded to the nearest five minutes therefore cannot fix the D60 lagna, and a time recorded to the nearest hour cannot fix anything in this chart at all.
That is the honest constraint on this page, and it is stated up front rather than buried. If your birth time is approximate, the D60 you get is one of several plausible charts, and a rectification is the prerequisite rather than an optional refinement. The wider divisions, D3 at forty minutes a part and D9 at about thirteen, remain usable where the D60 does not.
BPHS ch. 6 assigns the D60 past-life karma and the totality of life, and in the Shodashavarga weighting scheme it carries the highest weight of the sixteen. That is the classical position, and the reason the division is treated as the last word on a placement whose dignity is contested across the other vargas.
This page computes the division exactly and reports each graha's D60 sign, house, degree and dignity. The amsa deity and its benefic or malefic quality are computed by the engine too, from the Phaladeepika ch. 6 list, and they are carried in the full kundli rather than on this single-chart page. What is stated here is what the arithmetic settles.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.