I am looking for a starting load for 25/20 SS with breach seated 90 grain bullet
using 300 MP powder.  Am using 300 MP with good effect in my 32/40 and want to try it in smaller case.  Loads will be used in Winchester Hi Wall and Stevens 44 1/2  I am using mag. primers in the 32/40 should I also use mag. primers in the 25/20 SS?  Thanks Charlie

Isn't this one of those gotcha getcha things? I believe the 25-20 SS case actually has more capacity than the 25-20 WCF but that the heavier loads for the WCF have a higher pressure than was intended for the rifles chambered for the SS. This should mean that for the same load the SS would have a lower pressure than the WCF so starting with the low WCF load should be just fine and go up from there a bit. Of course, since you are using a high wall and a 44 1/2 you can go places that someone with a Ballard or a 44 cannot go but you probably just want to get into the 1400 to 1600 fps range anyway.

In my 10 twist barrel with a 115 grn  bullet I have had good success with 8.2 gr #9 and 8.3 grns of LiLGun. I tried about the same with MP 300 with not so good accuracy.