Calling for up to around 1/8 of compression of the charge is pretty normal for larger amounts. But on only 9 to 10 grains there just isn't all that much powder to compress. So more like some amount up to around 1.5mm or 1/16" would be fine.
The issue with only barely touching the charge is that with vibration the powder can settle and compress itself and that can result in a slight air gap. Especially if poured in with just a small funnel. So that's why compressing the load is a better way to go.
With barely room in the casing for 10 grains of black a slight air gap isn't the total end of the world. That's a light charge as these things go. But even a slight gap will up the peak pressure over what you'd get from a correctly compressed charge. So it's best to do it right in the first place.
If you want to run a lighter charge then you will want to fill the casing with something to ensure the charge is still compressed. Or you'll want to seat the bullet a little deeper to do the same thing. Someone mentioned a felt lube wad. That's not a bad idea. But you'll want to use something like a wax paper or similar separator so the lube in the felt does not affect the powder in contact with the wad. Or instead of a lube wad look at doing your own lube cookies. Or you can use a filler such as oat bran or Cream of Wheat cereal to replace the unwanted part of the charge. If you use a filler it goes in over top of the powder just before the bullet. Don't mix it in with the powder.
The issue with only barely touching the charge is that with vibration the powder can settle and compress itself and that can result in a slight air gap. Especially if poured in with just a small funnel. So that's why compressing the load is a better way to go.
With barely room in the casing for 10 grains of black a slight air gap isn't the total end of the world. That's a light charge as these things go. But even a slight gap will up the peak pressure over what you'd get from a correctly compressed charge. So it's best to do it right in the first place.
If you want to run a lighter charge then you will want to fill the casing with something to ensure the charge is still compressed. Or you'll want to seat the bullet a little deeper to do the same thing. Someone mentioned a felt lube wad. That's not a bad idea. But you'll want to use something like a wax paper or similar separator so the lube in the felt does not affect the powder in contact with the wad. Or instead of a lube wad look at doing your own lube cookies. Or you can use a filler such as oat bran or Cream of Wheat cereal to replace the unwanted part of the charge. If you use a filler it goes in over top of the powder just before the bullet. Don't mix it in with the powder.
