I hate those brain freezes!
Can you measure the length of the armhole and when you get to 4.5", start the bind off at the neck edge? If you bound off a few sts to start the armhole, measure straight up from there (not on a slanted line along the raglan edge). If the armhole starts with the raglan dec, measure from that first decrease, straight up until you have 4.5".
Since it's the right front, you'll be binding off at the beginning of a right side row.
It's a very sweet pattern.