One softstart manufacturer has given a rule of thumb for PFC as sq root of 3xVx0.8xno load current .Similarly is there any rule of thumb for starting across the line/star delta?.
You are mixing issues here.
PFC capacitor sizing can be estimated, but that is for all intents and purposes a SWAG* without specifics of the motor. As mentioned, many (if not most) capacitor manufacturers will provide a chart by which you can estimate the PFC capacitor sizes, but the charts make assumptions from generalizations about the motors. This however has nothing to do with starting.
Using capacitors for trying to reduce starting current is required to be even more specifically engineered for each specific application and something highly discouraged by most responsible engineers. The only advantage it offers over more conventional and legitimate forms of starting current reduction is cheapness, and that is rarely a valid engineering concern, especially when there are very significant risks to this method. The biggest problem with it, and one often ignored by proponents, is that the timing for switching out the capacitors is CRITICAL because the amount of capacitance is huge and if left in after the motor reaches roughly 80% speed, runs a severe risk of over correcting and creating voltage spikes, torque shock, all kinds of potentially damaging consequences. Just for you do do more effective searching, it is referred to as "Capacitor Assisted Starting".
*SWAG, if you are unfamiliar with the term, is short for
Scientific
Wild
Ass
Guess