Sending packets requiring Instants

[XenoKovah]

If I'd like to send an LL_PHY_UPDATE_IND to a peripheral to change to 2M PHY, I need to set an Instant when that will take place. What is the best way to do this? I'm guessing that querying the connEventCount from the python code and setting some value at some point in the future would either be too slow, or would require a bunch of binary search trial and error to get to the right minimum reliable Instant. Is there (/ should there be) a way to ask the firmware to fill in a "current connEventCount + X" value where the python side can set the X to 6 or 10 or whatever it wants?