Musar do Wyjścia 4:18

We have a tradition that after Yitro, i.e. Yeter, or Re-uel, converted to Judaism, his name was changed to יתרו to indicate that he had accepted the 613 commandments. If this is so, why was his name changed to one whose numerical value is 616 instead of 613? It seems to me that after Yitro converted to Judaism he was only entitled to a name whose numerical value is 606, since by converting Yitro assumed only an additional 606 commandments. He had been obligated to observe the 7 Noachide commandments already prior to his conversion. We have the example of Ruth the Moabite, whose name was changed to רות=606. Ruth's new name symbolised these 606 additional commandments she assumed at the time of her conversion. Yitro therefore should have been called תרו. He had the letter י added to his name as a special acknowledgment of the efforts he had made to find the true greatness of G–d and His Name. Even the letter ו in his name represents an acknowledgment of his contribution to the Jewish people through his suggestion (18,21) that Moses appoint heads of thousands, heads of hundreds etc. We have his name on record as merely יתר in Exodus 4,18. Rashi (18,21) points out that 600 judges were installed when the heads of thousands were appointed; the appointment of the heads of of hundreds involved another 6,000 judges. Appointing heads of fifty involved the appointment of another 12,000 judges, and the appointment of heads over each ten Israelites involved the appointment of another 60,000 judges. The whole world wonders why Rashi gave us a lesson in arithmetic here? Were we not able to figure this out by ourselves?