How do know when there is enough curve in a good board, or perhaps when is there too much. It's simple really. Just look.
The ideal environment for this consideration and the creation of such form is better done with the accurate lighting and minimal distractions in the peripheral vision as one crafts in a good shaping bay.
What is more important is ones ability to see things when they look right. I often challenge people to trust their ability to size up and feel a good design. Years of holding boards looking at a freshie or your mates,or perhaps an "old faithful" in the carpark or on the shore. Under sunny skies or at your local favorite shop the sense of what you feel, see and sense, has been framed by your fascination over time.
If you shaping a board or just handing you leash-less friend his board in the shallows keep training your senses, its part of being a surfer that is earned and can be trusted as its enhanced.
Enjoy your water