I have been using the 450 grain lee with IMR SR 4759 with pretty good sucess. It is also a bulky, fast burning powder that gives great reduced loads with cast bullets. My #1 458 likes these loads (450 lee with 26-30 gr.) to the tune that 3 of the 5 shots at 100 are often touching. I sent Accurate an email with basically the same question you are asking and their response is as follows:
Kirk,
You can reduce the load down as much you need with XMP5744.
However, even XMP 5744 will become inefficient at too low performance.
This means that you will observe some level of unburnt powder.
You will have to experiment yourself, and optimize and compromise between all factors I am attaching the published information from our comprehensive no 2 guide (392pages).
Note: It will be a .pdf file -- (Adobe-Acrobat) If you do not have it yet,
you can download Acrobat Reader 3 or later version (Available Free), from
the website
www.adobe.com to view it
NB!!!! Always start at the minimum or "START" charge and gradually (2%
increments) work up towards the "Maximum load".
Regards
Johan Loubser
Ballistic Lab manager
Accurate Powders
McEwen Tennessee
The file they included is about two pages and though I don't know how to post it here, I can email it to you. Just let me know.
Enjoy that #1 458...