parshanut torah comment - What is R. Chananel's/R. Bachye's calculation of the years in Egypt?

In this answer I collected the opinions of various biblical commentators as to how many years the Israelites actually spent in Egypt. One of those views was that expressed by R. Chananel, as recorded by R. Bachye Ben Asher in commentary to Exodus 12:40. The line I want to focus on says:

ת"ל שנה הם נמנים מיצחק משעה שנולד רד"ו שנה הם שישבו במצרים מנין רד"ו

The 430 years are counted from Isaac from the time that he was born. The 210 years are that they lived in Egypt a count of 210.

We know from Genesis 25:26 that Isaac was 60 when Jacob was born:

And after that came forth his brother, and his hand had hold on Esau's heel; and his name was called Jacob. And Isaac was threescore years old when she bore them. (Mechon-Mamre translation)

And we know from Genesis 47:9 that Jacob was 130 when he went down to Egypt:

And Jacob said unto Pharaoh: 'The days of the years of my sojournings are a hundred and thirty years; few and evil have been the days of the years of my life, and they have not attained unto the days of the years of the life of my fathers in the days of their sojournings.' (Mechon-Mamre translation)

Now if we take the number 430 beginning with Isaac's birth and subtract the 60 years until Jacob was born we are left with 370 years. If we then subtract the 130 years of Jacob's life until he entered Egypt we are left with 240 years. Thus, the Israelites were apparently in Egypt for 240 years.

Whence cometh the number 210?

(The way that other commentators reach the number 210 is by assuming that the 430 years started 30 years before Isaac was born. Ralbag who also states in one explanation that the 430 years began at Isaac's birth does indeed say that the amount of time in Egypt was 240 years.)