Don't leave powder in the powder thrower. This dries it out and changes the speed. It also leads to the accident of assuming it is Powder A, and dumping it back into the Powder A can, when it is actually Powder B.
I have seen three rifles destroyed by this error. The references to only one can on the bench is so that the powder gets returned to the correct can.
If you wind up loading for both pistol and rifles, be aware that pistol powder is very fast and will blow up rifles. The most dangerous combination is owning both rifle and pistol powder of the "ball" type. The two powders look almost identical. The solution is to make a mental note to either avoid ball powers, or only use them for rifle or for pistol, but not both.
Another error is thinking you can start withthe Max load. Start with the Start load and work up in 0.5 gr increments. Each rifle is differnt. I saw a case where the Start load was much too hot for a rifle (M96 6.5x55).
When testing for a better load, work with one variable at a time. Load a range of powder charges, using same prmer, brass, bullet and OAL. Once you have a good load, try seating the bullet 10 thou longer and 10 thou deeper.
Keep a log book. I use 3 ring binders, with a page for each gun (not for each caliber).