WhelanLad, there are four variables that will have some effect on the accuracy and velocities over the time period you may be worried about.
When you make any changes to the load, such as primer lots, powder lots, bullet lots, and if you replace the cases. These can and will create minor issues, which can be easily overcome by slight alterations to the powder charge. You likely know this.
The next issue is how much you've shot the rifle. After 300 rounds, it's almost always a good idea to seat your bullet about .010 further out, to make up for throat wear, then again at 500 rounds, 700 rounds. After that, you may not be able to catch up with the wear enough to make a difference in accuracy, without removing the barrel, setting it back a few threads, and rechambering. You should still get acceptable hunting accuracy out past 1600 rounds.
The powder charge you found "acceptable" should still be acceptable, but may need a bit of tweaking after stock work or changing to a different stock, as long as the bedding is tight and true, without pressure points.
Some loads just seem to work well in just about any rifle. The load you list is one of them, as long as seating depth is consistent with the throat wear, in the 35 Whelan.
Several notable firearms authors have written articles about "favorite or pet loads." They usually state in those articles that these loads will be acceptable as is and, with a bit of tweaking, may even yield excellent results.
Ken Waters, from Handloader magazine, wrote a book called "Pet Loads" and a follow up Volume II.
I met Ken Waters, along with John Barsness, at Mike Venturino's "ranch" where he had a private range. I was invited to the "ranch" by Mr Venturino, after meeting him at a gun show in Montana a couple of decades back. Mr Waters and Mr Barsness had also been invited, as they were all long time work buds and friends.
The one thing they all agreed on, Mr Venterino was the Black Powder, go to guy, Mr Waters was the handloading expert, and Mr Barsness was a well respected colleague, who knew his stuff.
The one thing which became evident quickly, was they all had it in their heads that there were some loads, that just worked in every rifle acceptably, and with a bit of tweaking, very minor, could shoot as well as the rifle was able to shoot the components loaded.
Back then, the Juenke gauge was just coming into play, in the bullet making industry. Bullet jackets were OK, but not nearly as OK as they are today when thickness consistency is critical for good accuracy.
Varget is very close to IMR4064 and IMR4320, depending on the lot. I have Varget from when it first was produced, and some recent production from two years ago. The earlier lot is faster than the later lot.
Both Mr Barsness and Mr Waters liked IMR4064 in the 35 Whelen cartridge with 250 grain bullets in the rifles they owned and shot.
Mr Waters liked 52.0 grains of IMR 4064, over Remington 9 1/2 primers, under just about any 250 grain 35 caliber bullet. Mr Barsness liked the same recipe with one grain more. They both agreed the difference was their individual rifles.
It's the same with just about all off the shelf, factory loads. The manufacturers have gone to a lot of effort to find a load that will work well, with the components they're using, in a wide range of rifles. The loads might even be tack drivers in some rifles, but should be acceptable for shooting out to 300yds, acceptably, on a hunt, in 95% of the rifles they're used in.
