How to make response time from specific time - sound stimuli

I am working for the first time on psychopy.
I use a protocol with 3 sounds ‘A’ ‘X’ ‘B’ where participants have to choose (keyboard) if X looks like A or B.
I use 8 blocks of 12 randomized items and everything seems to work perfectly on my first tests. On the other hand, I want to calculate my response time from the appearance of the 1st vowel of my sound X. I calculated the appearance of this vowel for each sound but I don’t find the condition formula in builder.
Not sure to be clear, but I don’t want the response time to start with a condition which looks likes : “$X==finished”, but i’m looking for something like “$X== a specific time”, different for each item.
Thank you for your help !