There is a very quick way to mark and cut the hem. Most shirts have tails. Mark the one front side panel curving up at the sides. I prefer to have a chalk mark that is about 1 cm (1/2 in) long for the new length, then I mark underneath that for the hem allowance. Try to keep the same curve line as the existing hem. Cut from the front edge along the bottom (on the bottom chalk mark) and follow the curve up to the side seam. Stop cutting. the front section is now your template, so fold this across to the first half of the back. Cut to the centre back following the front curve. Stop at the centre back, and fold the front and half of the centre back across to second half of the shirt and the front and cut. Your tails should be perfect either side.
Hem in the technique you want. When creating a small hem, the hem allowance should ALWAYS be small else it will not curve properly.
Judith aka genie