Hayanaratna 2.164

Now count the days from a planet's direct motion up to retrogression. Count also from its day of rising up to setting. Take half of that number of days. On the day with that count, the strength is sixty kala (one-sixtieth part). On the day of retrogression and on the day of setting, the strength is nil. In between, there is proportion. If the middle days give full good strength, what do the days gone or still to come give? Count them from the direct day, and from the planet's day of rising.

Simple meaning: Strength of motion by days: full 60 at the middle of the days between direct motion and retrogression, and between rising and setting. It is nil on the retrograde and setting days.