It's constantly changing. In short, you multiply your number of MVESTS by the current value of "STEEM PER MVEST" (which you can get from steemdb.io).

To get from VESTS to MVESTS in your wallet, you divide by a million.

The number of SP that you see in your screen cap from steemblockexplorer also includes delegations, but you can get the numbers without delegations from steemdb.io/@ruzmaira.

It's not exactly right, but for a quick approximation in your head you can divide VESTS by 2, then shift the decimal point 3 places to the left or divide MVESTS by 2 and shift the decimal three places to the right. As time goes on, that estimate will get less and less accurate, though.