THB,
You really haven't given me enough specific information to be able to give you a starting point based on the caliber and load. Here in a nut shell is what you need to achieve. You need to work with center to center measurements. For instance, once you get the barrels permanently into the mono block you need to insure that when you put in the forend bracket that the C. to C. spacing is the same or just a little narrower than it was at the breach end of the barrels. The mid spacer will be just a bit closer than at the forend bracket, and the front spacer needs to be roughly 10 to 20 thousandths closer together than at the mid spacer. This assumes 10 to 12 inches spacing between the mid and front spacer. I like to be sure that the mid spacer is well out in front of the forearm so that I can tap into it for attachment of the front sling swivel.
This last measurement is the only one that is really critical and it sets the final spread at any given distance. You can go ahead and solder in the other spacers but be sure to keep the barrels on the same plane vertically. The front spacer should be made of aluminum for ease of adjustment, and held in with a hose clamp, then once you have it right, you make one the same size out of steel.
The other way to do it is a bit more complicated but will work just as well or better, and that involves making a steel wedge with troughs machined at muzzle diameter, and set up on a slightly inclined or wedge alignment. By moving it in and out you can then adjust your spread at the muzzles but the wedge needs to be soldered in while shooting.
There is a third way which is the one I use, but I do not recommend it to beginners, and that is
to make up the permanent front spacer but leave it a little "fat". I then shoot the gun, un-sweat the spacer, take a bit off the spread, and re-solder it. I do this till I get the spread correct and only then do I worry about getting the final vertical adjustment corrected.
You must always worry about the getting the spread right first. You can adjust both ways to some extent at the same time but the spread has to be achieved first, then worry about getting the elevation of both barrels the same. If you get really lucky, both will come into alignment at the same time, but it doesn't happen often that way.
I have also found that the larger calibers are the easiest ones to regulate, it's the smaller and "hotter" calibers that are very "tweeky" and need to be shot and adjusted many times. I have regulated a double with as few as 20 rounds but that is a freak occurrence. Most of the time on average, it will take 50 to 100 rounds to get it right. If it needs to be parallel out past 100 yards, you can easily double that amount. Good luck on the new gun. Bob