Pedersoli Baby Rolling block, 44-40 bore diameter??

Looking at a pedersoli baby rolling block in 44-40.

But wondering what the true bore diameter is. All the info I can find on it says the same thing.

(Cut rifling .004" deep, 1-36" twist, 6 lands measure .424" and 6 grooves measure .432".)

Is it me or is my math wrong? if you go by the .004 deep rifling from the .432 that gives you .428.
Or .424 lands, .004 deep, gives you .428?

Any one own one of these that could verify the bore diameter? Confused!

Grooves are on all sides of the bore, gotta add the groove diameter twice. Another way to look at it - the radius is increased from bore to groove by 0.004 and the diameter is thus two radii.