Finding the LNBF target on offset dish with basic tools

[RimaNTSS]

LNBF's phase center should be 1/4 - 1/2 inch below plastic front cover.

Yes, approximately it is like that, but, unfortunately I have no formula how to get know Phase center's location precisely. For example, when this question was asked to Inverto Black Ultra producer answer was: Phase center of IBU is about 5mm behind the white plastic cover.