If your using Nosler data they used 24" barrel and it's pretty old data been used  since manual #5.  I know 20 let you seat out to 3.000", I have model 24.  You might want to look at data that seating bullet out vs Nosler @ 2.800"
This is something that may help
http://www.6mmbr.com/gunweek095.html
Hodgdon has data for 284,  139gr Hornady @ 2.915" start 52gr/H-4350 @ 2840fps and max of 57gr/H-4350 @ 3079fps.  It's not problem but 22" barrel.
 My 284, I'm  using 52.5gr/R-17 @ 2983 fps with 160gr AB and it long throated, 27" Lilja barrel.  I have other hunting rifles so 284  is just one of my hunting rifles and haven't shot anything lighter than 160gr.
I think been few article on that model 20 and if you could get hold of Mel he may have some loads.
Well good luck