Also probably worth mentioning... I think your number is actually right, but its interesting to note that you can also figure it out from the opposite direction as a percentage of new steem power.

You simply have to earn X% of new steem power where X is your current marketshare of existing steem power.

It is easiest to see this if you assume we start with 100% marketshare of SP. How much of newly created steem power must we earn to keep our 100% marketshare. obviously 100%.