I look at:
- Line from eyes to front hub, if it passes well forward of the bar tops, stem (or top tube) is too short.
- Imagine rider is in the drops, forearms horizontal, body in a low aero position, really hammering, if elbows are well behind the knees (just overlapping is normal), then stem (or top tube) is too short.


By my rules of thumb, that stem + top tube is just barely long enough for you.