GO Big-big and then pull ALL the slack out of the chain. Does it look like you have enough to add 3 more teeth?
The chain should be , as I understand it, 1 linkset (+any addition portion thereof) extra to considered proper length.
Many chains are longer than that. You basically have to look to know for sure.
If you have the chain set for the 28, you shouldn't have any problems with the 25. Look at all 11-34, 12-30 etc. the cassettes. The only time you would possibly get any droop was on small-small and you're not supposed to use that gear either (assuming you have a triple).