Yes, this is correct, about 8% of what SP you had at the time you set a powerdown to start. I intend to pile on the SP as quickly as I can get my hands on money to do so. I have about 500€/month coming through my mining, paid out every week, that's going ALL IN to SP, and I figure once I reach 2000 steem I will start a new power down. It's like a giant capacitor for my financial circuits. It is a very useful tool for helping maintain and disburse a big surplus so that you keep accumulating it.