+1

The length of the steerer tube determines how many (if any) spacers you must use. There isn't much room for adjustment, because if you have too many spacers the top locknut will not thread on. If you have too few spacers it's possible for the locknut to bottom out on the steerer tube making it useless. Threaded headsets usually have anywhere from 1-3 thin spacers. Sometimes cable hangers for centerpull/cantilever brakes are in there too.