A couple of clarifications. YYTZ, yes, sorry my phrasing was confusing.
And Shmoel is right. If it is provable beyond question that your maternal grandmother and her mother were Jewish, then your mother was Jewish, which makes you 100% Jewish. But I would not want to presuppose the response of a rabbi or beis din you consult with, who will scrutinize the evidence regarding your status, and who may, MAY, require a giur lechumrah.