Ok.. "prof" nb01 is here to save the day! Hahaha.... Think most of the ladies here have given good explanations but I am more long winded ok? Hee...
Firstly, are you trying to detect LH surge or luteal phase? Those are 2 different things but linked. Which is why it can get confusing.
Luteal phase is the back pack of our menses/cycle after we O. Depending on individual, the luteal phase can last from 12-16 days. Anything < 10 days is known as a luteal phase defect and can affect the chances of getting pregnant which is why it is good to know your luteal phase.
Although the luteal phase varies in length depending on individual, for each of us, it will always be consistent throughout all our cycles (but maybe +/- 1 day). For example, my luteal phase is 14 days and will be 14 days for all my cycles. The reason behind why we have irregular cycles is due to when we O. We can O earlier or later and this affects our cycle length.
For example, my luteal phase is 14 days and is 14 days for all my cycles. Since I have irregular menses (35-38), my O can be from CD21-CD24. But of course, it can also be earlier or later depending on our bodies as our bodies are strange creatures. Like this month, I O-ed super early on CD18 which means 18+14 = 32 days, which as you can see, is quite far off from my normal cycle length.
Since the luteal phase is also consistent throughout all our cycles, knowing it also helps in determining our O. As the longest part of the luteal phase is around 16 days, we start testing OPK on -17 days. But as I said, this is a guage coz if I had followed this, I might have missed my O for this month since 35 - 17 = CD18. So I always test once I see sticky/slightly stretchy EWCM and much earlier than the recommended -17 days. But basically, the rationale behind testing -17 days is from the concept of our luteal phase and not just a random number.