400 Nitro,
Yes I agree that the original velocities were taken from pressure barrels which were usually longer and showed a higher figure than what could be obtained in the shorter double rifle barrels.
But as I'm sure you know things don't always go to plan.
A good example of this is with mickeys load for the 470.
The original ICI load showed 2125 fps from a 31 inch barrel.
Mickeys rifle regulated at a higher velocity than this in a barrel which would most likely be at least 3 inches or more shorter.

Nitro X,
The bead MUST nestle in the bottom of the vee or the shots WILL go high.
As 400 said you should see the entire round front bead but not the blade.

I would not worry about those shots crossing at 25 yards.
Get them close at 50 then check them at 100 and alter the load to suit those distances.

If the standing rearsight has 100 engraved on it it means that the STANDING SIGHT IS REGULATED FOR 100 YARDS not the barrels.

As you said, you are quite close even with your first loads and this would suggest that there is a very good chance that everything will work out real nice.

So, sight with the bead resting in the bottom of the vee,
sneak the load up untill the individual barrel groups come together at 50 yards, then shoot it at 100 yards and see if they are apart or crossing and then adjust the load to suit.

You lucky bugger, I bet you are having heaps of fun with your almost new toy!

Scrubber or piggy's later on perhaps?