25.5 may not give a tremendous difference, or at least one that will make the plunge from C# to B too much better; if you don't mind heavy strings you can probably get away with it on that scale - I pull it off on a 24.75" scale (14-70 GHS strings, but even that is just because I like heavy strings) without a problem. You'd honestly want to start looking past 26.5" if you want that much of a difference from the scale length.
The reasoning I have behind this is that .75" is half of a fret. It's very intuitive to view longer scales as just having more frets on the bottom, and lines up fairly well when estimating string tension. At SevenString.org there's a topic with string tensions for tons of strings with different scale lengths, look that up and compare string gauges at that pitch and scale length to see what you're going for.
Jeff