We can measure the effectiveness of each PB window and compare them against simply spamming DK BS into EF Blitz. As of now Rising Phoenix is the only way to get Light Chakra by doing your normal form to form rotation except in a PB window which means that there will be a weaker (DK - Twin - Snap) and stronger (BS - True - Demo) variant. We'd need to average out the cost of obtaining a Light Chakra along with the payoff of Phantom Rush PB window to see whether that is better than straight up DK Boot EF spam.
So here goes:
Note: Base crit is around 1.4x multiplier with no extra crit stats. All BS in the equations are assumed to crit with Leaden Fist buff. TFC is 370 potency as of now and will be ÷5 to work out the average potency of obtaining a chakra (74 potency).
Dark Chakra window: BS (420+74) - DK (260) - BS (420+74) - EF (550~) - DK in Formless Fist (260) = 2058 ÷5 = 411.6 potency per GCD for that window.
Light Chakra window weak variant: DK (260) - Twin (230) - Snap (300) - RF (650) - BS in Formless Fist (420+74) = 1934 ÷5 = 386.8 potency per GCD.
Strong variant: BS (420+74) - True (270) - Demo (110 + 80*6 = 590) - RF (650) - DK in Formless Fist (260) = 2264 ÷5 = 452.8 potency per GCD.
Note2: Light chakra simply just wants you to go through the rotation. So the potency isn't as static as the Dark Chakra route. However if we average the strong and weak variant against Phantom Rush window we can estimate whether it will be worth it.
Phantom Rush Window: BS (420+74) - DK (260) - BS (420+74) - Phantom Rush (950~) - DK in Formless Fist (260) = 2458 ÷5 = 491.6.
So to work out if Light chakra is worth getting over Dark Chakra Spam window... we can average both variants against PR and see how it compares to Dark Chakra spam.
Weak: 491.6 + 386.8 = 885.6 ÷2 = 442.8 potency.
Strong: 491.6 + 452.8 = 472.2 potency.
We can see then that it is always worth going for Light and Dark Chakras over just Dark, because Phantom Rush window pushes the damage average above simple Boot and DK spam.
Note3: we don't know what the Two Seal Blitz will be, nor what chakra it will give. Therefore take all of this with a grain of salt. In general conclusion from what we know so far just spamming BS and DK under PB will result in a loss and it is better to go for Light AND Dark chakras for Phantom Rush. This could all change depending on any potency changes in the final build. Moreover, this is only in a vacuum without taking into account buff windows, fight design etc...