Your goal is to have the bottom of the threads on the steerer be at the top of your head tube when the fork is installed onto the frame. The reason you don't want a shorter steerer is because you don’t want the expander of the stem pressing against any of the treaded portion because it is weaker in that area and might bulge it out. Of course if your steerer is too long you can’t tread down the adjustable cup of the headset far enough. Any extra threaded portion can be cut off when installing the fork. It is sometimes useful to add spacers between the headset adjustable cup and nut if the handlebars are close to the height of the saddle.

Steerers usually have 50mm of threads. So what you do is add the length of the 3 components below the top of your head tube and add 50 mm. That is your ideal stem length.

In your case your head tube is 180mm, your lower headset stack height is between 12 and 15 mm (depending on what headset you choose) and your fork crown thickness (measured between the ledge of the lower crown race and the bottom of the crown) is going to around 20mm (sometimes it is 18mm). That sum is 215mm (depending of course on what components you actually chose). Now we add 50mm for threads and the total of 265mm is your ideal steerer length. Of course there is no 265mm steerer but you can cut a small amount off of the bottom of a 270mm one that you order. I usually figure I can cut off up to 20 mm on a steerer to custom fit its length.

Another variable is how far you insert your steerer into the fork crown. Lazy builders will leave it a mm or so inside so they don’t have to file any of the steerer off after brazing. I find it best for my students to leave about a mm more sticking out because it gives them something to push the end of the silver against while brazing. Many beginning builders have a hard time coordinating their left and right hand motions at the same time while brazing. A little assistance to keep the silver in the right place allows them to concentrate more on their flame motion.

It is possible to use a 240mm steerer with the result that a lot of threads will be inside your head tube but you don’t want to do that for the reason mentioned before. A good custom builder always selects the tubes that maximize their efficiency for a given frame.