You can't simply use less rifle powder due to "case fill" issues. When the cartridge case is only half filled it tends to lead to erratic burning. This because the powder could be pilled up at the primer end of the case, or at the bullet end, or both. No one knows. This erratic burn then leads to dangerous pressure spikes.
The way this is done is to use a very fast pistol/shotgun powder, like Red Dot, with a light bullet. It's not common anymore, but it's a well documented load.
Google "308 with Red Dot powder" and you'll get all sorts of hits. Since it's off the internet and not in a published manual, I highly suggest you wait until you find multiple articles recommending roughly the same Starting Load.
