Your downvote power is the same as your upvote power, it just does not regenerate as fast as your VP.
So it is currently approx. 0.08 up and down (in rshares) for a 100% vote. (on a post with more than 20 STU it is more than 0.08 due to the curve)
You can do 12.5 downvotes until your downvotes power is at zero and the next downvote would reduce your voting power instead. The downvote power is 12.5 downvotes which regenerate in 5 days, so you can do 2.5 downvotes every day for free.