I'm not sure if this forum section is only for Barb to give answers, or whether this question would be better off in the general help section?

But if I might:
My thoughts would be that I would need to know what the waist/hip measurements are , so that I could work out what the taper would be, and then it would be easier to know how much would be lost on the taper. I'd hesitate about splitting the 4 thread white line over 2 pleats in any case as you would still have the same tapering issue. I was mending an old kilt recently with a double line which was just the same size as the pleat width, and the original kilt maker made that pleat a little wider so that the tapering wasn't evident, all the other pleats were the same (narrower )width, it looked absolutely fine,and wasn't evident unless you looked really hard. I'll see if I can find a picture of it.
It's not unusual for the actual sett size when pleated to be slightly different , so don't worry too much about matching it up exactly, a slightly wider ,or slightly narrower band isn't really going to be noticed,and it does give more options for the pleating.