Albert Barnes Commentary Numbers 18:27

"And your heave-offering shall be reckoned unto you, as though it were the grain of the threshing-floor, and as the fulness of the winepress." — Numbers 18:27 (ASV)

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.