Have you emailed the bullet manufacturer or checked to see if they have a website with load data?
Since your manual is on backorder, they should have an interest in keeping you safe and happy with their customer service.
You may also find load data data from the powder manufacturer's web site.
Remember that heavier bullets take lighter powder charges to achieve the same performance. Remember that if you wind up having to interpolate between bullet weights.
Also remember that the cartridge length is intended to do three things, in INCREASING level of importance to safety.
1) ensure feeding through your action (least important for safety)
2) ensure the bullet does not jam up against the rifling.
3) set the base of the bullet such that the volume inside the case is appropriate. For the same bullet weight, if the same overall length (nose of bullet to base of case), a long-nosed bullet or deep hollowpoint bullet will have its base deeper than a blunt or flat-point solid bullet OF THE SAME WEIGHT. That is, a smaller volume, which will lead to higher pressure.
What I am suggesting with point #3 above is that you should calculate where the BASE of any bullet you seat is...and compare that to the location of the base of other bullets (presumably with a different nose profile.
The simplest and safest thing to do is to load in the medium-light range of powder charge at first and work up loads over a chronograph, watching for signs of high pressure (or low pressure). If you don't have a chronograh (or access to one), gather as much information from bullet, brass and powder manfacturers (see my first paragraph) as you can.
Good luck.
Lost Sheep
p.s. I just looked at Berger's web site. I did not find data for the 280 Remington. In am confident that an email to them will get a response. It would have to if they are responsible and if they have the data.
What powder(s) are you thinking of using? Sticking with the H4831 (which I missed seeing on my first read-through)
