Bike Forums - Determining fork travel on unknown fork

Hi everyone,
I want to use an old fork I have as a starting point to know which new fork to get for an ebay frame I bought. I don't know what the frame was intended for... 80mm, 100mm, or 120mm forks.

But, the old fork is a Rock Shox Dart 1 circa 2007 off my wife's 2007 Trek 4500 WSD. Seems to be either 80mm or 100mm but I can't tell for sure from web searching. It might be adjustable between 80/100 with an internal bumper installed or left out.

I measured from the top of the dust seal to the bottom of the fork crown, and that was 100mm. Can I assume it's a 100mm fork?

Or, if there is a bumper inside to make it only 80mm travel, would it sit like an 80mm fork or a 100mm fork?

I've used it on a hard-tail I'm building up, and the bike rides nose-down with this Dart fork. So I'm pretty sure I want a longer fork when I buy a good one, but I don't know where I'm coming from. (Besides the fact that the fork is shot and it's 1/2 way through its suspension all the time I'm riding it.)

I think I'm going to go for a rigid fork, so the length is pretty important. Any advice how to know what I'm currently using?