I see that many people have issue with trying to make it perfect... That's totally against what we were expecting :C
I was expecting people to get annoyed when they are forced to be exact (and it seems you are) so we made some margin of error so you can do task "good enough".
Generally MY system for building planets is to make planet with mass of 100. so for 30% of hydrogen you collect 30 units etc.
I guess we took that into account, but failed at informing that this feature exist. Maybe there should be some signal that planet is good enough and can be placed on orbit.
But I really thought that people will not try to be perfect...
I added note in game description ;)
Thank you for feedback!