Let $T$ be an $\omega$-stable theory, and $P$ a strongly minimal, $\varnothing$-definable set inside of a saturated model $M$. Let $a \in M$, $b \in P$ such that $tp(a/b)$ is $P$-internal, that is, $a \in dcl(d ,p)$, for some $d$ independent from $a$ over $b$, and some $p \in P$. Let $m = MR (tp(a/b))$, then there is a formula $\varphi(x,y)$ such that $\models\varphi(a,b) $ and for every $b' \in P$, if $\varphi(x,b')$ is consistent, then $MR(\varphi(x,b'))=m$.
I am not sure how to construct this formula, as I don't think we can just pick some $\varphi$ witnessing the Morley rank of $tp(a/b)$, since we need the rank to be preserved as we change the parameters (without leaving $P$). I believe it is necessary to use the fact that $P$ is strongly minimal in order to be able to express the Morley Rank of a formula as an elementary property of its parameters. This would be possible if $a \in P$, but we only have $a \in dcl(d,P)$ for some tuple $d$ (I don't see how this could be enough). Am I on the right track? This is mentioned vaguely by the author, but I've been so far unable understand the full argument.
Any help is appreciated!