I worked out a while ago that you needed about 15,000 XMG to generate one XMG per day - however the rate has dropped a lot since some changes in the spring - I'm no longer able to work it out as it fluctutates a little
It's now 20,000 XMG (in chunks between 750-1000) to generate one XMG per day.
What you're experiencing here is the variable interest rate of staking coins. As far as i can tell there wasn't a change in the code, rather than the amount of staked coins.
I haven't looked into the code, but in general there's evidence that the amount of staked coins directly influences the interest rate.
I've started tracking the magi metrics sometime back in February and as you can see on the following graph the more coins are staking the higher the interest rate:
https://www.mupload.nl/img/x5ycuqs.jpgTo know how many coins you need to gain 1 Magi per day is quite simple to calculate:
coins needed = 1 / ( stakeinterest / 365 )
you get the current interest rate with the "getmininginfo" method of the magid client. (Users of the GUI-wallet can type in that command in the last tab).
Just notice that numbers changes quite frequently, so you might take a average of the returned values.
I smoothed the following graph and your values seem to fall into that range;
https://www.mupload.nl/img/pryprba.jpgFrom my experience staking 100-200 coins is the minimum you should try. below that you probably run into the problem that your coins age faster than they gain weight. As mentioned by others before the sweet-spot seems to be around 500-1k coins