Hello,
When I imprint t-shirts I use my fingers to gauge the position down from the bottom of the collar (three fingers) If you pre-press your shirt for three seconds(which you should) then when you lay your fingers on the tee you can make a mark with your finger so you will know this is the top of the transfer.
Most t-shirts from factory have a crease line in them so you can align them on your press. If you are imprinting a crest in the heart area, use the three fingers down from the bottom of the collar (this marks to top of the crest) and three fingers in from the crease line (this marks the left side of the crest).
Now depending who makes your shirt, you may have to look closely to see the line. According to my manufacturer, they say the center crease line is standard on all t-shirts made.
Hope this helps.