another question: whats the correct way to calc tax IN the payment?

If the base payment is 707 pre tax then tax upfront is:
( 707 * 36 payments ) + 1.1k inceptions * 8.875 (nyc)= $2355

Dealer is telling me tax in payment is 783... which seems to be too high:
707+tax= $770 + mf on this amount $5 = $775 not $783 - small but curious why the diff

And on MSD, do you give 707*7 or 783*7?

thanks!