Should "Where do you use to be?" be accepted? I wrote that and lost a life. Perhaps it is correct, "use to" (present time) does not sound that good really but had worked better in past tence I suppose ("Where did you use to be?"). Hope some native English speaker can help me here.
"Where do you use to be?" doesn't make sense. "Where did you use to be?" is different from "Where are you, usually?". "Where did you use to be?" would indicate that the habit is in the past and not going to resume in the foreseeable future. You would ask this to new neighbors or a new coworker and they would tell you about their old address or job.
"Where are you, usually?" or the slightly less formal "Where are you usually at?" would be asked if you want to ask about an ongoing generality/habit/routine, one that might not be being followed at the time of the question.
Plejer traces it's meaning back to (Middle) Low German to a word (plejen) meaning "to be responsible for"/"to be obliged to", and yes another meaning in Danish is to take care of a sick person but it doesn't share the other meanings of the English word "care" which would be at work in your construction. For those the Danish equivalents are omsorg (n. attention)/bekymring (n. worry)/ bekymre(v. worry).
I have been searching through Google books looking for an example of "Where do you care to be?" meaning something other than "Where do you want to be?" and I have not found it. I have been crawling through texts from the early 1800s to the 1960s and have not found an example, but I have also had to skip a bunch of legal examples. Could you please link to a source that uses "Where do you care to be?" to mean "Where are you, usually?" and not "Where do you want to be?"