Sforno on Exodus 12:40/ספורנו על שמות י״ב:מ׳

1אשר ישבו במצרים שלושים שנה וארבע מאות שנה, the sum of 430 is arrived at by commencing the count from the time G’d took Avraham out of Ur Casdim in order to conclude the covenant of the pieces with him. During that conversation with Avraham, G’d had specifically taken credit for taking Avraham out of Ur Casdim, (generally understood as saving him from the fire of Nimrod’s furnace when Avraham had been a voluntary martyr for his belief in the G’d of heaven. Genesis 15,7) This is the reason why the author of Seder Olam, an ancient historical text, describes Avraham as having been 70 years old at that time.