A point to stress here is the need for a chronograph in developing loads for double rifles, particularly vintage Brit examples.

The normal pressure signs for modern bolt rifles are not obvious at the lower limits of older break-open actions. To continue adding powder to bring the barrel-groups together with no attention to pressure is folly, and the only surrogate for pressure available to the layperson is existing published velocity.

So working up to achieve regulation (with medium/slow to slow powder) is fine, provided that the published velocity is not exceeded. If that velocity is reached before regulation is achieved, then change powder or projectile. It is potentially dangerous to continue increasing the powder charge.

If regulation simply can't be achieved after all load possibilities are tried, then the barrels need re-regulation. If this step is taken, at least the rifle can be regulated for a sensible pet load with consistently available components.