Building תצ״ט ופלגא

In Yahel Ohr, at the end of Parshat VaEra the GR"A brings an explanation as to how it is built.  He credits the sefer Ginat HaEgoz.

If you add the gematria of all the Hebrew letters up until צ, we get to 495.

That is to say

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90 = 495

Then we are left with the last four letters of the aleph-bet.

100 + 200 + 300 + 400 = 1000

Elef is then counted as Alef, as one.

495 + 1 = 496

And then we add the sofit letters:

ך = 500

ם = 600

ן = 700

ף = 800

ץ = 900

Therefore:

500 + 600 + 700 + 800 + 900 = 3500

By the same principle, we divide this by 1000.

3500 / 1000 = 3.5.

496 + 3.5 = 499.5