Dont do it. You dont want anything other than the real thing in there. Dump the powder, weigh it, put it back in the hopper. Do it 25 times. By the 10th time you will know if your charge is correct and go with it.
You are seriously overthinking this. The only part you have to handle with care is the primer. If you hope to practice priming with spent primers prepare for frustration. They will be deformed and will not easily go back into the primer pocket. Just use a live one and take care to have it square in the seating tool and the case properly centered in the case holder. If you feel excessive resistance back off and check the alignment again.
Primers are set off by being struck. A slow push is not going to ignite them.
Smokeless powder is not the same as black powder. BP is an explosive, modern smokeless powder just burns really quickly. It will not ignite unless you introduce a source of ignition. Don't smoke while you're making ammo and don't do it by candlelight. You'll be fine.
To check your charge, throw a charge and weigh it. Then put the powder back in your hopper. Repeat at least 5 times before you even think of seating a bullet. The charges should all weigh the same. If they don't you need to fix your powder thrower before moving on.
As you work through a batch, re-check the weight of the thrown charge every 10-20 rounds to make sure it is still correct. Toss the weighed powder back in the hopper - do not try to get it back in the case.
I've nearly gotten the bullet seating and crimping down. Next steps will be seating the primer and loading the charge.
I'll probably seat a few spent primers just for practice/giggles. I also need to practice to determine if I want to use the primer seater built into the press or the handheld seater.
That will bring me to weighing the powder and charging the cartridges. I'd like to practice those steps as well before going live. I was thinking a fine grain sand like beach or play sand would be a good substitute? I know sand and powder will not have the same weight/volume, meaning I can't set up with sand then substitute with powder without making adjustments. I want to ensure I am getting consistent results from the equipment and from me before I pour the good stuff. I also know I will need to verify (with a scale) every so many pours. I intend to use an older RCBS Uniflow Powder Measure.
Anyone done this before? A quick Google search did not answer my question.
Yes, I am probably over thinking it. I covered in another thread that over thinking things is one of my specialties.