First off, welcome to the forum, and welcome to reloading.

You're off to a good start, with a manual, and asking questions. That's good.

There's more than one way to skin a cat, and more than one technique to reloading. So some of this advice will seem to contradict what others have said, but as long as you're using a safe method and verifying your work, any of these methods will work.

When I'm developing a new load, or working with a new rifle (or barrel), I like to start with components that have worked for me in the past under similar specs (I have the advantage of reloading for almost 40 years). It's also important to use components that match the intended purpose for the gun. I don't test match bullets in big game rifles, and I don't test hunting bullets in match rifles. Then I look at the load range from min to max on that specific bullet/powder combo. So for your Varget and 155 gr Hornady, the Hornady manual lists 35.9 to 44.9. Normally I take this range and split it right down the center. In this case that is 40.4 grs. That would be "my starting" load for making test loads.

On a 308 sized case, I go in .5 grain increments, and I load 5 of each. So I would load 5 rounds at 40.5 (simpler to go in .5's), 5 at 41.0, 5 at 41.5, etc, etc, on up to 44.5 grs which is where I would likely stop. I actually write the bullet weight, powder and charge weight on the side of the case with a fine tipped "Sharpie" when I seat the bullet. Then I take all of this test ammo to the range and start shooting at the bottom loads (40.5), and shoot my way up to the top loads. Now having said that, if along the way, I start seeing any pressure signs, I'll stop shooting the increasing increment loads until I figure out what the cause is.

The idea here is that hopefully, one of these loads will "show me" something good. I shoot all 5 of each test load into a separate target, or area of a large target, so I can keep them and take notes on them. IF one of these loads shows me something good, like a group size I'm after (depends on what the rifle's for), then I will make special note of that load recipe, and I will go back and load 25 more of that exact same load. If I get more than one "good" group, then I'll generally work with the higher charged load, for velocity and trajectory reasons.

My "requirements" for "good" group also depend on application. For a match or prairie dog gun, I require 5 shots cutting into same hole. For a big game rifle, 5 shots must all go under an inch. Tighter is better, for sure, but that will be "good".

If that 25 round batch also shows me the same good group, then and there, I have "finished" load development for that recipe. That load recipe now goes into my permanent reloading notebook.

If, on the other hand, I don't see anything good in that initial test range batch, then I change ONE thing at a time, and start over again. Never change more than one variable at a time, or you can overlook combinations that end up being "The One". So, only change seating depth, or only change powder type, or primer, etc, etc, and start all over again when you do. For hunting rifles I start off seating bullets to 20-30 thousanths off the lands, assuming it will still feed through the magazine on a repeater. For match or even prairie dog guns, I'll start with them seated much closer, like 10 thousanths off the lands.

The reason I don't start off on the very bottom of the load range is, most powders will burn very inefficient at minimum load. You get erratic combustion, and shots likely will vertically string, or can even "shotgun pattern" all over the target. Wastes more time and components imho. Even at "mid-scale" loads, you often don't start to see good groups until you move higher up in the pressure/combustion scale. This is especially true as cartridge case size gets larger, and powder burn rates get slower. But I stick to .5 grain increments so I don't "fly past" a good load without knowing it.

And on a side note, for test development, you need a true shooting bench type setup. You are basing recipe decisions off of the groups you shoot, so you need to be very precise and accurate in your shooting technique. That way you don't miss a great recipe due to a bad hold/shooting position. Shooting off of a car hood with a bipod will introduce far too much "wiggle" in your gun and give you false group sizes. You need to eliminate the shooter as much as possible when testing loads. We're testing loads, not testing shooters. :)

And I've seen guys scorch their paint jobs with powder burns, crease their hoods with bullets, and even crack their windshields from muzzle blast.

Just my 2 cents worth anyway.