Hi Matt,
I wish I had one of those Hornady 300 grainers to look at. I just opened up my Horanady reloading manual and they call for a 1.600 overall length for the cartridge with this bullet. The max load listed is based on this length. I think that will be with the lower canelure or the one that puts more of the bullet out of the case.
I think (you should confirm this) the upper canelure is for loading the bullet in 444 Marlin's. You are probably not looking at a dual canelure for performance sake but so one bullet can be used in 44's and 444's. Check with your calipers but the 1.600 cartridge length is probably as good as you are going to do in 44mag.
As far as reading pressure by primers goes, I have never been able to do it in handguns running normal magnum pressures. In my experience, if you are showing primer flattening or cratering, you are already way way over pressure. The problem is there are so many variables to what pressure signs show up first and what they mean, you could go nuts trying to figure it out. With jacketed bullets I'd stick with the reloading manuals recommendations. This is also to be specific to bullet, brass, primer and firearm. The other issue is that what may seem over pressure in one gun will appear quite normal in another. I recommend you get some reloading manuals and read up on all the signs of being over pressure. Others on this forum know more than I do and may have some better advice.
Your 300 gr Hornady bullet is very good as jacketed bullets go. Because of its hard jacket and size, work up loads slowly. Big pressure spikes can come quite suddenly with this combination. Hope this helps.
God bless................. Bill M