Tom,
One of the problems with computer twist formulas is most consider plastic tips part of overall length. But the tips are so light it doesn't work that way.
The guy who developed the twist formula used in most programs today, the late Don Miller, eventually figured out all that. The JBM Ballistics formula includes this factor, so works far more accurately with plastic-tips:
https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
