Reckoned unto you - Or, by you. The Levites were, of their tithes, to pay tithe to the priests, just as other Israelites paid tithe to the Levites.
Related Commentaries of Numbers 18:27
Numbers 18:27
27 And this your heave offering shall be reckoned unto you, as though it were the corn of the threshingfloor, and as the fulness of the winepress.