As reflected in comments, there's no real "rule" here (though there's a tendency for -or to occur more often in words with Latin roots). So basically, you just have to learn them.
But things aren't as bad as they appear. Not only is the -er form more common in established words - it's far more "productive" for new terms. Also, as RegDwight points out in this ELU answer on the subject, there are many words where either spelling is acceptable (adviser/advisor, convener/convenor, etc.).
So instead of having to learn every word separately, all you have to do is remember those where only -or is acceptable (which as of today, includes administrator).
In short, slow as it might be, the general trend is towards -er. Adopt that as your default, and with any luck by the time you need one of today's more obscure "-or - only" words, the -er form will be acceptable!
EDIT: As @Anixx correctly points out, strictly speaking there is no currently productive suffix -ter in Modern English (the only instances where it's recognized as a meaningful "morphemic element" are laughter::laugh and slaughter::slay). This question and answer address the -or / -er distinction.
I would use "Build answer sheet" or "Assemble answer sheet" or "Format answer sheet"
or "Insert markers".
Also you have misspelled 'containing' in two places.
Best Answer
According to Collins COBUILD Advanced Learner's English Dictionary, you can use -ish in three different situations. The third usage matches your scenario:
-ish is added to words referring to times, dates, or ages to form words which indicate that the time or age mentioned is approximate.
Considering the meaning of '-ish' i.e. about/somewhat, when it's used with time and age, to answer your specific question, I guess you should ask "when do we use about/almost?"
I think generally we use about 50 years old when,
We're not sure whether a person is 50 years old or not but his looks make us think of this approximate number. Like what witnesses do while giving descriptions to a police officer.
When we know the number but we want to round it up. Like 50 years and 2 months or 49 and 8 month but it's easier to say 50 (Here I'm not thinking of the real concept of rounding up in maths because it's based on clear rules already).
So when you say "fiftyish" you mean "more or less than fifty".