What part of taxes was given to the churches in Kievan Rus?

Exactly 10%. That’s just not taxes. This was in Byzantium and was called “ruga”. Vladimir, who baptized Russia, according to historians, borrowed more the institution of “tithes” from the royal capitularies of Charlemagne in 774-790. Byzantium later introduced tithes in the eleventh century, which was a tax in kind for the harvest.
In Russia, tithe was not a separate tax. The prince was in charge of the treasury. From it, 10% of income (not taxes from the population, but from the income of the prince and military production) went to the church.