Modifying from V-neck to jewel neckline
I'm knitting a sweater for my husband ("Geoffroy" in Norah Gaughan's "Men" issue from Berroco). The only problem is that he doesn't care for the V-neck shape, and so I told him I'd make it as a jewel neckline (regular T-shirt style neckline). While I'm tempted to just have a go at it, I'd appreciate any general tips for this type of modification.
I've currently got 100 sts on size 5 needles after binding off for the armholes (the pattern is the same all the way across). Is there a general rule for a T-shirt style neckline? They always seem to call for a big bindoff, and then decreasing 1-3 sts each side on each row for some number of rows after. Is there a formula for the % of the total sts I should bind off? Or how many I should decrease and for how many rows thereafter? I'm generally doing worsted weight sweaters, so I'm hesitant to try and convert that over to a smaller needle.
Thanks much for the help!