number - Were the there only 55 שמיטה years between the Exodus and the First Temple?

The sefer Sifsei Cohen (not to be confused with the commentary on the Shulchan Aruch) on VaYetzeh, Beraishis 28:13 writes the following:

ואמר הארץ אשר אתה שוכב עליה לך אתננה...עוד אתננה אתן נה פי׳ אחר נ״ה שמיטות אחר יציאת מצרים נבנה בית המקדש

He writes that the First Temple was built 55 shmittas after the Exodus. How does this work out?

Now, according to מלכים א, ו:א, the First Temple was began 480 years after the Exodus. It was completed seven years later.

Now 55 x 7 = 385, which is much less than 480. Perhaps he doesn't mean to count all the seven year periods, but only the actual Shmitta years observed. Now, the Shmitta cycle only began 14 years after Israel's entry into the Land. That was 54 years after the Exodus. 480 - 54 = 426 years of Shmitta observance before the First Temple. That number is still much larger than 385. Even if we were to add the seven יובל years in 55 shmittas, would only bring us to 392 which is still far less than 426.

Can anyone figure out what the ש"ך is talking about?