Sforno on Numbers 18:27/ספורנו על במדבר י״ח:כ״ז

1ונחשב לכם תרומתכם, to ensure that blessing will be present in your homes even though we are speaking about something which has been measured, something not ordinarily subject to blessings. Our sages had gone as far as saying that when someone prays for G’d’s blessing on things he had already counted, measured, or weighed, this is a futile prayer, i.e. he uttered the Lord’s name in vain. (Taanit 5) This תרומת מעשר, tithe of the tithe is exempt from such restrictions.