Assuming max jump distance is about 200, try something like:
Add random(-200, 200) to VAR_Plat
After that add sub events to verify that VAR_Plat is within playing field:
VAR_Plat < 96 -> Add 200 to VAR_Plat
VAR_Plat > 864 -> Subtract 200 from VAR_Plat
Then you can create the platform.