Plastic tips don't matter much in overall length for twist-rate, because they're much lighter than even hollow-points without any lead inside--and usually "pointier" than even essentially closed hollow-points, like those on many match bullets, or Berger Hunting bullets.
The late Don Miller, who devised the twist-rate formula that both the Berger and JBM programs are based on, eventually decided plastic tips don't count at all in overall bullet length when using his formula. But he may have modified that if he'd lived longer.
JBM's twist-rate formula has a factor for plastic tips, while Berger's doesn't. When using the Berger formula I add 1/4 the length of the plastic tip to the jacketed length of a bullet to calculate stability, resulting in pretty close to the JBM results.
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