Best to use the specified value by unit manufacturer. Unless you have the equipment to measure the current phasing between run and start (auxiliary) windings you cannot really tell the correct run capacitor value. I use closed loop high bandwidth hall effect current sensors on run and start windings, with hall effect sensors displayed on multichannel oscilloscope.
Measuring compressor power factor is a method but it is not super accurate. Make sure you are measuring current on compressor common lead and voltage across Run winding for PF reading.
Another rough way is to measure run winding current and start winding current then calculate sqrt( run winding current ^2 + start (aux) winding current ^2). If this result is close to compressor common wire current, then you are close to correct Run capacitor value. When current between run and start windings is 90 degrees phase shifted, indicating proper run cap mfd value, then the square root of sum of squares gives the net common wire compressor current.
For example, run winding reads 10.3 amps rms, start (auxiliary) winding reads 6.9 amps rms, common compressor wire reads 12.4 amps.
sqrt ( 10.3A ^2 + 6.9A ^2) = 12.4 amps.
There is also variance on optimum run capacitor depending on compressor mechanical load. Typical average outside temp 80-90 degs F running current on compressor is between 60% and 65% of RLA nameplate rating if you have nominal AC voltage level at unit. Phoenix or Las Vegas in summertime will be 85% to 90% of nameplate RLA. Compressor load also goes up if condenser coil is clogged up with dirt.
The run capacitor and run currents have nothing to do with Easystart as soft-starter only applies to startup period.
The measurement of phase shift between run and start windings during startup period is even trickier. The phase shift will change as rotor slip changes. It is fairly flat between 20% and 80% of startup period.
I don't think you ever want to get close to 90 degrees current phase shift during startup period. This would produce too much startup rotor torque and may damage compressor, like snapping Oldham ring that keeps orbiting scroll from rotating. With too large a start capacitor the compressor rpm's will go from 0 to about 3500 rpm's in less than 0.1 seconds.
Best way to select a hard start cap value is to look for 0.3 to 0.6 seconds of startup time period.