I agree with everything John says. Forget about loading to another cartridge level what ever they may or may not have been. Different shaped and capacity cases with different diameter bullets and seating depths just does not translate to reduced load levels in another case and bullet size. Just select normal full performance loads for your 416 Rem Mag and then create reduced loads to the level you want in that cartridge. Dropping down in bullet weight will help in this regard if you want to use jacketed.

Personally to shoot a lot of 404 from my rifle I find cast bullets and light charges of fast powder (shotgun or pistol) the way to go. Easy on the shoulder, easy on the pocket, easy on cases and easy on the gun too. At this level or loaded up close to jacketed velocities, cast loads are effective on game.

A chronograph is really essential to be able to see what you are achieving and to predict bullet performance unless you have enough opportunities to shoot game to enable you to appraise performance in the field.