What is the reason the Kohen would pour this blood in that particular place, and what is the Torah source for this? The verse states, "And all the remnant of the blood of the bull he shall pour onto the base of the burnt-offering Altar, which is at the entrance of the Tent of Meeting." The burnt-offering Altar is the "Outer Altar", positioned in Temple Courtyard. The "Tent of the Meeting" was designated the inner Hall of the Tabernacle while it traveled in the desert, and then this term was transfered to the Temple Hall.
Which part of the Altar does the Kohen encounter first when he exits the Temple Hall? The western part. Since the rule is that one may not pass up the opportunity to do a mitzva, even in order to do another one, he pours out the blood right there.
Art: Thomas Sidney Cooper - A Bull And Cow, Two Sheep And A Goat