Any deviation from the published reloading data puts us into unknown territory.
All the warnings say to definitely begin any creative loads with the low end of powder and then try to figure things out from there.
Since it's not possible to know what the real pressures are, without some expensive gear, we have to judge by the looks of the fired brass and the velocity, as measured by a chronograph.
Since you aren't changing things by a huge amount, only a slightly shorter cartridge, velocity should give you a good idea of what's safe.
And how far to go to get the load you actually want.
Do you have a chronograph?
Lock the doors, they're coming in the windows.