As suggested try and find out the load the rifle was regulated for. If a modern DR the gunmaker. If a vintage DR the barrel flats may prove useful and give you the bullet weight and cordite charge.

The cordite charge can be calculated and converted to tell you the velocity the bullet needs to do.

Otherwise, it is sometimes possible to guess the load from the usual ballistics the DR chamberings had.

You will need a chronograph to work out when to stop increasing the powder charges when building up loads. ie waiting to see pressure signs on cases when reloading for a DR will mean the loads are already over charge.

Generally you stop around about when the original NE velocity is achieved.

Choose a bullet likely to be similar to the regulating bullet. Woodleighs are often designed for this purpose.

Research some loads other people have been using, and any velocities achieved. DOUBLE CHECK any loads from the internet. Loads can be obtained from reloading manuals, NitroExpress,com!, other sources, BGRC members and similar clubs etc. If the powder is different from the one you want to use, try looking up relative burn rates eg on the ADI site and estimate a similar starting load.

Use a starting load less than the ideal powder weight and work up.

Use a filler if necessary in big cases to avoide hangfires and excess pressure spikes.

Don't use bullets such as monometals, H-mantels, Partitions Barnes X in a double rifle. IMO anyway. Use a lead core bullet with more "give".

Start shooting loads of 2x2 loads, one shot from each barrel then repeat. Mark which bullet hit where, eg R1, L1, R2, L2.

Find out if the barrels are shooting high or low, crossing or not. Crossing might have the right barrel bullet impacts landing to the left of the Left barrel impacts.

Sometimes its a matter of test and see. Are the bullet impact "groups" getting closer or further apart as the powder charges are increased? As said you want the right barrel shooting more or less to the right and the left to the left, ie the barrels should not be crossing. Velocity has an effect on this.

Ideally you don't want the bullet impacts to cross at all, but hit roughly say an inch apart parallel at say 70 yards or 100 yards.

Remember as you shoot say the right barrel the gun may swing up and to the right before the bullet exits the barrel. Up and to the left for the left barrel. Increasing velocity will get the bullets out before as much movement occurs. Picture an X when the barrels were regulated and this was taken into account by test shooting the DR by the gunmaker.

Obviously you also want the load to also be individually accurate - ie just like a single barrel firearm, but this one is using one load for two different barrels so possibly a compromise. However my Tikka shoots almost one hole 2 shot groups from each barrel, about 3/4 of an inch apart - up and down - for an U/O DR. My .450 has a considerably different "groups".

Also shoot a DR without the DR touching a rest. Eg rest the DR on your hand and your hand on a rest. Otherwise the impacts may be thrown out. Shoot a big bore from a standing rest to lessen recoil effects.

Also adding a scope, or taking one off can have an effect.

You may have to play around with different powders, bullets, cases, primers etc to get it to work.

Sometimes a rifle needs regulating. As said some cheaper rifles have wedges between the barrels at the muzzle and half way along to allow re-regulation by the user. My Tikka/Valmet is such an example. Others need to be re-regulated by a competent DR gunsmith, the solder removed etc. A much bigger job!

The above is just some comments from me, and I am not an expert at it at all.

As said short cuts are finding out the makers loads, or ballistics or loads of the same cartridge by other owners.

Tell us what you have and other information if you will.