You can adjust a whoopie sling down to around 12 inches long on each side. This would give you a bare minimum tree span of around 11ft with an 11ft hammock. In real life practice, I would suggest a minimum of about 12ft to allow enough room for suspension and strap adjustment.
Check out the Hammock Hang Calculator for more details at https://theultimatehang.com/hammock-hang-calculator/