I presume you are shooting this bullet out of a Ruger # 3 chambered for 45-70? The NEI mold/bullet looks pretty similar...at least to me.... to the 535 grain Lyman, Postell bullet. I used to shoot this bullet(Lyman) a fair bit in the mid-1990's, in a 45-70 McMillan Sharps. Mostly black powder, but occasionally with smokeless. In this case, SR 4759, IMR 3031, 4064 and Varget. Don't recall the exact load data, but, fairly close to those shown on the Beartooth site. You could likely hunt with this bullet, but, if heavy hunting bullets are your preference, I found that a paper-patched, 450 grain Lyman Whitworth, or similar bullet, was a better choice for game. Not so much for deer, but moose, elk, etc. I sized this bullet to .459" (patched diameter). The Lyman bullet features a large, flat meplat, rather than the roundnosed contour of the Postell. So, it seemed to transmit a good deal of shock, even at extended ranges. Other than Goex Ffg or Pyrodex RS, I believe IMR 4064 was my powder of choice for this bullet. Average muzzle velocity was between 1100-1200 fps.
Though possible, I think you would have a hard time getting 1500 fps out of this 535 grain NEI bullet. At your seating depth, there isn't much left for the powder charge. Are you attaining this at the muzzle? With which powder? If so, your data could be very informative.