An the actual shape depends on several factors:
- The loading profile
- The weight and thickness of the material
- The elasticity of the material
- The presence of, elasticity of and load distribution on hems
- The angle at which the guy line leaves the tarp
- The wind pressure at any given time
In practice, you can cut an approximate cat cut, then adjust the position of the peg to suit. Elasticity of the material should take up slack.
If the cut is too aggressive, more tension will be born by the hem. If it is too straight, the hem will be slacker.
I wonder why nobody has written about incorporating shock cord into a hem. That would have elasticity and bear load, especially beyond the elastic limit.