Have a Trek 26" aluminum frame that I picked up pretty cheap. It's a new 2013ish 4 series, but it takes a standard 1 1/8" fork.

My question is--it originally was spec'ed with a 100mm suspension fork (did not come with a fork when I got it). I'd imagine the axle-to-crown was somewhere around 475mm. How important is it that I get as close as possible to this number? It's just going to be a knock-around bike to ride with the kids and such, but I don't want to screw up geometry so bad that it becomes difficult to ride.

Plus a rigid 26" bike will take me back to the types of bikes I rode as a kid, so I thought it'd be a fun build.

I did this exact thing with a Giant Boulder. The Surly 1x1 fork is designed around 100mm suspension. Check the spec and have at it. If sag is taken into account, Surly should be right there.

That's actually the exact fork I was looking at. I'll check with the LBS as every place online shows it out of stock.

You can replace a suspended 26" fork with a rigid 29er fork, put a bigger wheel up front. It's called a sixty-niner

Not really my cup of tea. I think there was a reason that never caught on.

Probably around 470mm give or take.

Suspension forks are supposed to have a certain amount of sag when you ride the bike, if a 100mm fork has 25% sag, the fork will have a axle-to-crown measurement of about 445mm.

20 to 25% sag is commonly used on suspension forks.

A suspension corrected fork with 440mm to 455mm axle-to-crownis what you'd want.

A few mm's won't be noticeable.

The Kona P2 is another fork you might want to look at: https://www.bikeman.com/KON-P226RD.html

'Suspension Corrected forks' are Made to Not drop the Head tube angle and alter the steering.