Yet another question about limiting

I am limiting my first quad, and I want to make sure that my understanding of limiting is correct.

I printed the limiter target with my chosen base quad. The first step with visible tone was 251. In a perfectly matched negative, the first visible tone would occur in step 254, since 255 always prints at paper white due to super high negative density. So the limiter needs to change the ink curves so that the ink signature (combination of inks) used to produce gray tone 251 moves up to 254. I suspect it’s done simply by stretching the ink curves.

I plan to limit in PPEv2 using the Start Point method. The default start point is 255, not 254 which is a bit confusing, since 255 can never be the start of tone. OK, so I’ll set the Start Point to 251. But there’s a magic number 5 which the instructions say should be subtracted: 251 - 5 = 246. I would like to understand the reason for the 5. Fudge factor?

PPEv2 limiting is different than the PiezoDN limiting instructions. The PIezoDN limiting instructions are actually probably a little better than doing this as a starting point limit in PPEv2 because the PIezoDN method (using the curve adjustment tool + QTR droplet) will maintain the full blocking of the 255 white point patch.

best regards,