what would be a decent 1200- 1400 fps load with a 500gr. cast bullet out of a 458mag

Here's the best one I have that might come close to fitting your needs:
These are fired from a 21" bbl....your results may vary.
Using an NEI .45 cal. 500gn "Badger-Browning" mold, and 30:1 alloy gets me 491 gn. bullets. Sized .459, lubed with "NRA-alox".
WLRM primers, 50 gn. IMR 4895.
Mv(ave.)=1377fps. Es=16. This rifle has a Millett red dot sight on it. @ 50 yds, shooting over a coupla sandbags, these loads consistently chew up 1" target dots creating a single ragged hole.
Hope this helps....
Cheers,
Don

don edwards thanks ,thats what i,m looking for ,some place to start