You might find out whether you have sharp step chambers or gradual forcing cones, but the actual length of the chambers should be entered into the equation. You are looking for expert advice, so you should share that here. I have no idea why you would search out a 1250 fps load to shoot in an 1895 gun. There are other alternatives.

My chambers have the gradual forcing cones.

I was seeking out NiceShot factory loads since RST can't get the raw shot. So, I contacted Dan at NiceShot Inc., and he stated that this loading was his "standard" for low-pressure duck shells. ( 1 1/8 oz., 7900 - 8000 psig, at 1200 fps.) So, I just went with his advice.