Hi @rasa, yes the technique is correct although I would question that the data you are working with is truly a fixation and not simple raw gaze data.
You are quite correct that the values of saccade velocity are typically higher, so I would check the source of how you determine fixation…can you pass me the link and I will take a look for you.
I found this tool which may be of used to you? https://www.tobiipro.com/about/partners-resellers/app-market/fixation-detector/