Oh, sure. Sat down to write the ransom note, knowing exactly how long the Ramseys would be at the Whites' party and being certain they would not come back in a few minutes because they forgot something.
What did the intruder do, call over to the Whites' house and say, "OK, all ready now, send them home!"
If the killer is someone who knew the family --- an employee or associate of John's, a relative of someone who worked in the house -- they could have easily been privy to the Ramsey's plans for day. The intruder didn't have to know "exactly" how long the Ramesys would be gone -- he only had to know they were going out for a party and it is reasonable to assume they are going to be gone for at least an hour.