I checked technical information for a few films and it looks like below 1/1000s should not be a problem. Just to be sure I can use flash at full power what should lower SB-28 flash duration, based on tests I found, to ~1/300s. Please see attached (I did not know that for TRI-X Kodak recommends +10% development at 1/1000s).
If I will take care about high intensity reciprocity failure and gel flash to lets say 4500K (bc I rarely take photos at noon). Will that be OK to test film speed / development time in that environment if I'm going to take "real photos" outside in the sunlight using results of the test performed inside with flash like described above?