After years of shooting out across wide open fields I know just what you mean about not being able to see your impacts. Most of the terrain where we hunt/shoot is clay or some variance of it so it dosn't show much for dust on impacts.
We got our gong set out at 500yds couple months ago and had a go at it with our favorite hunting rigs. I can honestly tell you it was pretty humbling at first. At first we didn't look up anything as far as holds we just pretended it was a hog out there and felt the need to bust it. Then after several misses, in basically the same conditions as you mention, we consulted our load dope and went at it. It definately opened up our eyes to a new world no doubt.
Good luck on your upcoming ranges as now it is on, and you will be walking it on out to test your limits.
My scope's a Nikon Monarch 4-12 Adjustable Objective. The first time I tried this I had a Leupold vx1 4-12. The Leupold vx1 wouldn't track properly (I'm sure the upper end ones would). I asked on this website about a scope for under $500. The Monarch was recomended by several people (post-"need a good scope").
I like this scope. It's clear, bright, and the tracking is accurate. I've read several good reviews about the buckmaster 4.5-14 side focus from Nikon as well. I chose the Monarch for the better glass.
I was pleased with the 12x magnification at 500 yards. While I haven't shot further yet, I think the 12x will be plenty out to 800 or 1000. I'll soon find out. The paralax adjustment is a must.
The rifle's a sporter weight, stock winchester 70 pre 64 in 270. I bedded the action and turned the trigger weight down. I also developed the handload I was shooting using the ladder method (post- very first ladder test).
I used a benchmaster rest to shoot from. I've yet to mount a bipod and try field positions. I'm looking forward to this.
I had put my gong on a bit of a slope. While I couldn't see my impacts, I could have drove out and looked at the holes in the ground. Next time I go out I'll back up until I can't hit the target any more. I plan to put my target on a slope again, so I can at least go see where my shots are hitting.
ITs cool to listen to the gong when its hit. The thing to do like someone else mentioned is to get confident at 500. Then keep backing it up 100 yards till you get confident at hitting it. Pretty soon, 500 yard shots on deer size animals is pretty easy, even with regular rifles and 3-9 duplex reticle scopes. Practice and knowing where your bulelt goes is the key factor, nothing else.