Accurate Arms lists 5744 with the Speer 350 Grain Lead at 1912 fps with 49.5 grains and at 2173 fps with 55.0 grains.

http://www.accuratepowder.com/data/PerCaliber2Guide/Rifle/Standarddata(Rifle)/416Cal(10.57mm)/416%20Rigby%20page%20329.pdf

I just ordered up 400 of these bullets and need to round up some 5744.

I would try XMP5744. I have no data, but my guess would be about 30 grains to start. But don't take my word for it, check with the manufacturer.

Red label, do a search on my name and you will find some posts of mine on this topic. XMP5744 or whatever it's called seems to be the standard without the need for a filler. Unfortunately, it is hard to find, so, I did some research and found a load for my 500/416 double rifle using VV 130 I believe. The loads for the 500/416 are very similar to those for the 416 Rigby (which I also have). I too wanted reduced loads for lead bullets, and found them. The VV130 does require a filler, but this is is 5 second job. Of course it helps that my wife likes quilting and so we have some of this stuff around. Now, my reduced lead loads do NOT shoot to the same point of impact that the full blown hunting loads do. BUT, they impact with a huge wallop. I was shooting at 5/8" steel plate at 50 yards and the lead bullets put a huge dent in the front and a big bulge in the back! Let me know itf you can't find my posts and I will send you my load. I explained the interpolation/extrapolation process I used to develop my loads in my above referenced posts.
Peter.

Zir has the link correct, call them too and they will give you more loads.