irrespective of what your vimsottari dasa says, or what transit of a planet says at that moment, varshaphal, which is based on sun's position gives you close enough result.

this is calculated in simple way.
see at which exact degree was sun at your time of birth.
every year when sun comes to that exact degree+minute+second , cast a horoscope with planets in their current position and ascendant at that time.

If varsha ascendant is 6,8,12th house to your birth ascendant, then that year will be full of troubles, else it'll be good.
Also dasas are calculated in same ratio as in vimsottari.
365/120=x will give the multiplication factor for sun's period x*6, moon's period x*10, mars period x*7 etc
based on those planets position in that varsha chart, you'll see their results in those periods