Don't make your readers hold parts of the sentence in their head. Reorder or split sentences until it can be avoided. In other words, use a really small buffer.
"You must fill form 331 unless you are over 60 years old, in which case you must fill form 445."
"You must fill form 331. If you are over 60 years old, you must fill 445 instead."
"If you are over 60 years old, you must fill form 445. Otherwise, you must fill form 331."
You might have noticed that the last version is an if-else statement.
"You must fill form 331 unless you are over 60 years old, in which case you must fill form 445."
"You must fill form 331. If you are over 60 years old, you must fill 445 instead."
"If you are over 60 years old, you must fill form 445. Otherwise, you must fill form 331."
You might have noticed that the last version is an if-else statement.