I had a Brno ZKK602 which had a 23" barrel. I had no trouble loading it to 2400 fps without having to load heavier than the manuals said, from memory I loaded 98 grns of IMR4831 for about 2420. That was enough

, but showed absolutely no pressure signs. It is possible to load the Rigby round to almost 416 Weatherby speeds (& pressures) with perhaps another 10 grains of powder than that, so there is a fair bit too play with. A change in barrel length will not alter your pressure but you will need a stiffer load in a short barrel to give the same velocity as a longer barrel, however, as I've said, pressures are so slight with factory or similar reloads this is easy to do with no stress. Another 2" of barrel with this load might get you another 40-60 fps, conversely, 2" less would give a similar amount less.
Steve