Full disclaimer that I'm just getting into sewing (a few months in), but I think you could get one of those frabic markers that washout with just water.
- Mark where you want the v-neck.
- Cut the hoop down the middle to where your 2 lines intersect.
- Fold where you made the marks on the shirt.
- Sew down the fold to make a hem.
The hem won't be the same width, so if that matters to you I don't think my idea will work. You could minimize how noticeable it is by match the thread color with the shirt. Another option is to make it really visible and make it a design in the shirt. Even if you make a mistake you can just say you meant to do that.
