Hodgdon #27
45-70 Government Marlin 1895
350 grain Hornady RN
Winchester brass
CCI 200 primers
54 grains H4198 MAXIMUM
2191 fps in a 24" barrel
In most Marlin 1895G rifles this load will certainly exceed 1950 fps or even 2000 fps, and will most likely achieve 2050 to 2080 fps. And yes a chronograph is very useful in working up a load in an 18.5" barrel that is published from a 24" barrel. And more common sense than fear, as well.
You continue to grasp at straws. There is no way of really knowing you are within the 60,000 PSI limit that SAAMI establishes for the 30-06 SPRG either without measuring the pressure. That sure hasn't stopped people from working up and using published maximum loads for the 30-06. That is a red herring.
Your quote of a 25% increase in pressure with a 4% increase in the charge assumes that someone would exceed a published load for the Marlin 1895. No one here is suggesting that would be done except you. So it is a red herring as well.
There is absolutely nothing wrong with using the Hodgdon 45-70 data for the Marlin 1895, whether it has a 24 inch barrel or an 18.5" one. Adapting load data to individual rifles and components is a very real part of reloading and it is done all the time. And done safely by the vast and overwhelming majority of those who do it. You can't control what any one person is going to do. But it is just plain wrong to suggest that a 350 grain 45-70 load achieving over 2000 fps in the Guide Gun is necessarily dangerous.
The Marlin 1895s that have been properly used with 40,000 CUP loads are not the ones that are going to end up on the gun racks in questionable condition.
