You have to purchase at least 2 SP worth for our system to delegate it (that's a blockchain limitation). I don't know how much you need in order to transact, but I suspect it depends on what percentage your voting power is currently at, so I don't think there's a simple formula. For guys who like to play with large numbers, here's a post that talks about required mana to post/vote right now: https://steemit.com/test/@tabris/hf-20-test-posting
You can check how much mana you have on steemd.com (i.e. steemd.com/@boyasyie)
You have to purchase at least 2 SP worth for our system to delegate it (that's a blockchain limitation). I don't know how much you need in order to transact, but I suspect it depends on what percentage your voting power is currently at, so I don't think there's a simple formula. For guys who like to play with large numbers, here's a post that talks about required mana to post/vote right now: https://steemit.com/test/@tabris/hf-20-test-posting
You can check how much mana you have on steemd.com (i.e. steemd.com/@boyasyie)