Just had an email conversation with Ben Amonette from Alliant. Here is what he told me for reloading the 257 Roberts with RL 17...

RL 17 and IMR 4350 are very similar in burn speed. If you have a starting load for IMR 4350, you can use the same starting load with RL 17. The max loads may not be exactly the same but they will be similar. Thanks for your return note.

It looks like an interesting powder, and they may have data for the 257 Roberts soon. But, at least this will be a starting guideline.

Start looooow,and work up!

I haven't used RL17 in a Roberts. Just comparing velocity in a 7x57 I found RL to be about 1-1.5grs hotter than IMR 4350.