centrifugal advance curve MSD Distributor

I just installed a MSD pro billet part number 8534.
I swapped out to the light springs but did not change advance stop bushing because I am having a little trouble understanding why I should change it.
I set my timing at 38 degrees with the lightest springs and the car rocks, I'm just wondering if it would run better with the red as opposed to the blue stop bushing or should I go the other way ?
choices
red- 28
silver-25
blue-21
black-18
anyone have a setting that worked well in there 10/1 with ross flat tops
Block was decked cleaned etc..
J headed with 2.02 1.60
mild bowl ported heads
484 comp cam that comes in around 2500 rpm
850 double pumper
The motor should be around 400hp or so the way its set up
Im useing a 727 trans and a convertor that flashes around 2800.

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