"Where are you at half past five?"
Translation:तुम साढ़े पाँच बजे कहाँ होते हो?
The use of होते here indicates this is habitual, so it's like asking "Where are you at half past five [every day]?" Do I have that right? Would the sentence be meaningful without the होते -- something like "Where are you at half past five [today]?" or would you need to use the future tense for that?
You're right. If you wanted to talk about today, you could say -
तुम आज साढ़े पाँच बजे कहाँ होगे।
In the habitual sense, I think it would be होते हैं, but होते हो seems questionable.