I'm fine with that ruling, but I'm not sure that's the way that spell is intended to work.
It's effectively a Summon spell. like Mount or Summon Monster, but it summons a weak invisible force construct.
The 35' range is referring to where the creature was originally summoned. After it exists, it can move at it's base speed for as long as it is conjured into existence.
At least that's how I understand it. If your ruling were accurate, Summoned Monsters would vanish after getting a certain distance away from the caster.
Again, however you want to do it is fine with me. I just wanted to explain my interpretation of the spell.
The floating rune spun once in the air and drifted off, copies of itself seeming to split from it and fade from sight as it went.
The bodies began dragging themselves away, their clothes peeling off as they went. Potentially useful items began drifting out of windows to arrange themselves neatly in the yard. Soon the entire fort was a bustle with activity.
Now everyone gets to find out why my familiar's name is Legion. :)
His one and only trick is the ability to cast Unseen Servant at will. Each casting has a duration of 5 hours.
Legion can create a lot of Servants. :)
Back at the old tower, Grey was the reason on one needed to do any chores. The whole tower was effectively haunted by a full cleaning staff.
The elaborate raised rune on Grey's mask disconnects from his face leaving the mask smooth and bare. It flies forward, the two potions in Grey's hands levitating and moving with it.
Cast Unseen Servant as Standard action. Move to square V17. Unseen Servant takes potions as Free action, moves to Hagren's side and administers the Restoration potion as a Standard action.