The printed Hodgdon Reloading Manual using IMR4064 has 42.0gr, 2554 fps, 43,800psi for a start level; and max load at 46.3gr compressed, 2767 fps, 59,700 psi for max load. This is with a COAL of 2.750" for Hornady 165gr SP, 24" barrel and Federal 210M primers.
I would use Hodgdon's information as the guideline if you have a newer rifle. If you have an old rifle that cannot use modern SAAMI load levels of max 60,191 psi chamber pressure, then go with the lower level from Sierra. The 43.5gr may/likely be an accuracy node load. You can use Hodgdon's Reloading information online here.