You don’t (or shouldn’t anyway) guess on these. Hisako’s rekkas are actually fully reactable in terms of how fast they are - if you focus only on trying to block them, then you should be able to defend against them pretty reliably.
Also something to note is that Hisakos that seem to be really good at parrying are almost certainly guessing (unless you’re using a broadcast move like jHP with Eyedol, homing puddle punch with Glay Glay, wind kick with Jago, etc.). Average human reaction time is 12 or so frames. So, any move with <12 frames of startup can’t be parried on reaction by a normal person. If she keeps countering normal pokes, then you can start testing her knowledge of you. I just played against a Hisako who seemed really, really good at parrying. Round 2, I started baiting parries out, sometimes by just taking a few steps forward and not doing anything else. Empty forward jumps tend to work, too.
Parrying is not a problem atm, it’s more how to deal with her constant pressure with rekkas and mixups.