Bike Build: Threaded fork measurements? What size do I use for my frame?

Headtube length + headset stack height (roughly 1-3/8")

More to the point, the Rush has a 1" headtube, so you'll need a 1" threaded fork. The majority of threaded road and track forks are 1" (threadless is mostly 1 1/8"), and will probably come with a nice long steerer tube - definitely longer than you'll need.

Threadless steerers are cut a lot longer than the headtube, because they have to stick way out of the headset enough to attach a stem. Sometimes people will leave a lot of slack on the steerer and block it off with spacers below the haedset's stem cap, so they can move the stem/bars up and down.

Threaded steerers don't work the same way, because the height of the stem/bars is is independent of the steerer length - the shaft of the stem determines the handlebar height. With a threaded setup, the steerer is threaded down into the steerer tube and then cut off just above the headset. There has to be enough steerer over the headtube that all the headset elements can be screwed on securely, but not so long that it's not clamped down tight. It's possible to use spacers on a threaded headset to take up excess stem length, but it's not as common as with threadless. If you use something like a cable hanger or a rackmount/light mount/bell mounted around the steerer, that acts as a spacer, and the thickness of the mounting ring needs to be calculated into the headset stack height.

In both threaded and threadless cases, the headset cups are mounted in the headtube.

As SkidMark notes, the calculation for a threaded steerer is (headtube length) + (headset stack height) = steerer length. Some people will advise the steerer tube should be a 1-2mm short of the combined height, so the locknut can screw down tightly. Headset stack height is the vertical measurement of all the headset bits that appear outside the headtube; it doesn't include the parts that are friction-fit inside the headtube. It's tricky to measure stack height on a loose headset, but most headset manufacturers publish their stack height. If you have a frame with headset cups mounted, and you stick a bare fork into it, you need a little steerer tube sticking out; otherwise, there's nothing for the screwed toprace/lockwasher/locknut to attach to.

According to SOMA's geometry chart, the 53cm Rush has a 110mm headtube. Once you've picked out a headset option (or a range of headsets; you can substitute a higher/lower headset to deal with steerer tube length), you'll have a steerer height range of 5-10mm that will get you into the right neighborhood. That's if you're looking at used forks; if you're buying new, you'll have to cut it or get an LBS to cut it for you.

Depending on the fork, you may also need to get the threading extended, so that it goes low enough to screw your headset together. This will almost certainly require an LBS, because the dies to thread steerer tubes are expensive; nobody but a shop will have them. A lot of shops are willing to extend the threading on an already-threaded steerer; it's easier to do, because the threading is already started square to the tube. But I've talked to a number of shops in my area, and I have yet to meet anyone other than a framebuilder who's willing to thread an unthreaded fork; even the framebuilders who were willing to do that weren't willing to thread a fork unless they'd made it themselves.

Since you have a SOMA frame, you could do the easy thing and buy a pre-threaded lugged-crown SOMA fork - $130 for black, $150 for chrome, in four different steerer lengths. They also have a straight-blade track fork - $150 for chrome only, three steerer lengths, drilled for brakes. You can get them for less from other resellers

To me, the SOMA forks look like the chrome Tange forks that are available everywhere. I've seen them used for as little as $20, and new for about $60. If your LBS doesn't have one lying around, they can definitely order one from one of the big wholesalers; they all stock them.