Useful calculator for 'ya--

Bicycle Trail Calculator | yojimg.net

With 25s all around my rig is a fairly generous (by modern standards), 60 mm of trail (57-59 seems to be what is most commonly seen on modern road bikes). Interestingly, with 28s all around my rig calculates to 61mm, which leads me to believe it may be as much as 62 with a 28 only on the front leaving a 25 tire on the rear. Some bikes from the legendary era may go higher but >63 would be unusual for most bikes.