A well-meaning fellow baal tshuvah informed me I should not say the brocha on the shel rosh of my ’gifted’ tefillin until I have the strap expertly measured to fit my head. It ’seems’ to fit fine (hits the bump at the middle-back of my skull). Must I keep desisting from this part of the brochu until a posek fits it for me? Thank you and L’Shana tovah.

The "bayit" should be completely above the hairline and the knot below the bone on the back of skull, preferably just touching it. It never hurts to show your tefillin to an expert, but you may (and should) recite a bracha if according to your understanding your tefillin sit right.