I read the description of the Lee Factory Crimp Die. I am concerned that it may be a variation on the roll crimp, which is not suitable for a straight walled cartridge like the 450 BM. The preferred crimp is a taper crimp to .474 to .475 inches.
If you want a firmer crimp, I and others use a side crimp (search around for information about it) followed by a taper crimp to remove the slight "belling" that the side crimp imparts to the mouth of the case.
This has come up before. Here is a link -
http://450bushmaster.net/viewtopic.php?f=8&t=15624.