The pit road properties are one of the toughest and inconsistent part of track building. Typically, the earliest pit stall location is the marker for where pit road speed starts. Guides will suggest to place the earliest stall roughly 4 meters after where you want pit lane to start.
In the track.ini, there's usually pit stall locations for well over 43 cars; for Pit Lane 1 and Pit Lane 2. Usually I assign the normal race stalls to Pit Lane 1, and use Pit Lane 2 as the definition for where pit road starts and ends. You can do that by setting the highest number stall in Pit Lane 2 to 4 meters after pit lane start, as mentioned above. I usually use stall_42. It's the same for pit road end as well, but use stall_0 in Pit Lane 2 to define where pit road speed should end.
Worth mentioning that using Pit Lane 2 means you'll also have to define that second pit lane, include .lp files and more lines in the track.ini. Most tracks already have this in, I would guess the same is true here too but just be aware if it is not.
Like I said though, it can be an inconsistent science. Sometimes the end results still isn't perfect.