What it is
Shashtiamsha divides each sign into sixty equal parts of 30′, half a degree each. It is the finest of Parāśara's sixteen vargas. In the vimshopaka scheme, which gives each of the sixteen vargas a weight, Parāśara gives D60 the largest share.
How it is calculated
Rule: the count starts from the same sign and runs in order; the zodiac turns five times across the 60 parts.
Example: a planet is at Aries 17°15′. 17.25 ÷ 0.5 = 34.5, so the thirty-fifth part. The thirty-fifth sign from Aries is Aquarius (two full rounds of twelve, then the eleventh). In D60 the planet is in Aquarius. At 17°35′, only 20′ further, it would be in the thirty-sixth part, Pisces.
What tradition looks at in it
Tradition holds that the shashtiamsha offers a glimpse of the imprint of past actions, the tendencies a person brings with them. Parāśara also gives each part a name. ePandit draws no good-or-bad verdict from these names. The plain meaning of karma is that today's actions shape the road ahead; D60 is there to understand, not to worry over.
How much the birth time matters
On average the ascendant moves about 1 degree every 4 minutes; some signs rise faster, some slower. One part of D60 is 30′, so a difference of roughly 2 minutes in birth time can change the D60 ascendant. Even the Moon crosses one part in about an hour, and the ascendant in about two minutes. In your chart, ePandit shows next to every varga how many minutes of birth-time change would shift its ascendant. Even hospital-recorded times are often a minute or two off. Without a precise time, give the D60 ascendant no weight; some astrologers use this very varga to rectify the birth time.
How to read it calmly
Read D60 as a second look, not a verdict. Tradition first studies the whole main chart (D1), then the running dasha. D60 only helps check whether the same theme shows up here as well. A planet's sign in this varga decides nothing on its own. D60 carries great weight in tradition, yet with an uncertain birth time it is also the least dependable varga. Read it last, and most lightly.