A simple multiplication factor should be more than accurate enough. Just divide the new time by the old time to get the factor and then multiply all your N+, N- times by that.(posted 8105 days ago)
Say the old recommended Kodak time was 6 minutes, and the new one is 7.5; then your factor would be 1.25.
Multiply your own calibrated times by that, and you should be close enough.There are some published results that seem to show there's a proportional relationship between the induction time (time to first appearance of a visible image), and the development time to a given density of ANY developer with ANY film. Whether that information has any practical use aside from inspection developing is another matter. This 'Watkin's factor' was used quite widely before the time and temperature method became normal practice.