Taper in the arm hole up to the shoulder of the t-shirt when cutting. For the kids tanks I bring it in by a little less than an inch.
For an adult I actually start the arm hole a little higher than it normally is on the pattern (at the armpit) so my bra won't hang out, and then taper it up so that it's in a little more than an inch at the shoulder.
Then I just turn under and hem with a double needle.
