With a red dot at 200 yards, I clamp a stick with an 8" colored aim point so that it is 3-4 feet above the target. Then I hold on the aim point. lob 5 shots out there, find the holes, and then adjust the aim point to group on the target. At 300 yards, the holdover is like 8 feet or so, can't remember, so I would need a sand bagged ladder and a stick and a couple of clamps to be able to position maybe a 24" colored aim point above the target. And it would have to be a dead calm day. You will need to choose an aim point color that contrasts with the background when viewed from the shooting table. Don't use a green aim point in front of a bunch of green trees or a brown one in front of a rocky hill. I usually size the aim point to be twice the diameter of the dot size, so, if one is using a 2 moa dot, you would want at least a 4" aim point. I use one much larger than that, but you get the idea.
Last Friday, when shooting targets at 100 yards, I tried holding 3 feet over a 24"x24" gong at 200 yards. This was just an estimated holdover, but it worked, I hit the gong. Not a good way to try to shoot groups at 200, but ok just for a fun shot.
The other thing you will need to dial everything in is a large light colored target backer, so you can see where you are really hitting relative to the small target. At 300 yards, I would want one or two 4'x8' sheets of cardboard.
At least that is the approach I would take starting out at 300 with a 9mm, doesn't matter if it is a rifle or a pistol, you are still just lobbing low velocity bullets out to the 300 yard target.