You're estimates are off by quite a bit. In micro buyin levels, you'll typically see players cashing (I.T.M., 'in the money') at a higher frequency than in the larger buyin MTTs (I believe from many going by the theory... play to get ITM... then try to move up the money ladder... quite often we're then in bad shape and unless we get lucky once the bubble pops we usually don't run that deep in it).
Another note here... usually the payout for just making it ITM (just making it past when the bubble pops) is only around 1.5x your buyin (or even less). Try looking thru the lobbys of tournaments that have been completed and you'll see this to be the case.
The best online MTT players generally finish I.T.M. at a rate of 11-12% (this seems to be the average amongst the top guys who play online tournaments, with avg. buyin $100+... same goes for the med.-stakes buyin MTT players ($26-$75).. avg. ITM is ~11-12%). Players with higher I.T.M. usually don't have a higher ROI% (<< return on investment). ROI is the number you should actually be more concerned with.
So... what's a decent ROI%? It 'depends', lol. It depends on what your primary motivation for playing MTTs is, what buyins you're playing, etc. (ie. if you're playing for the highest winrate/hr. then you'd obviously need a decent ROI% if playing the micro levels, because there will be many times that you'll play for 4hrs. in a tournament and not make much more than your buyin in return).
Some good (high buyin) MTT players only
have an ROI of +25% but at the same time they might play a very high volume of them. Others are +150%.
As mentioned above, you really want to shoot for the final table when playing MTTs. At first you might just want to play to get in the money which is fine but without a decent cash along they way it'll be difficult to maintain a bankroll, nevermind increasing it.
MTTs are a tough go... until you get a decent cash that is. This is why most MTT players suggest not playing an MTT that is more than 2% of your bankroll (many say only 1%).
I think if you just focus on improving your game along the way, gaining more experience all the time, play with good bankroll management, etc., the bankroll will take care of itself (in other words.... you don't need to calculate that you'd need to be cashing 40% of the time).
Sidenote.... for SNG play (single table SNG's that is)... 40% ITM is about what you'd need to be shooting for to be playign profitably. Anything below 35% ITM and I'm pretty sure you'd be running with a -ROI% (unless you had a disproportionately large number of 1st place finishes).
GL on the tables!!