I used to do it the same way as you, except I’d heat the copper a bit before pouring the DMT onto it. That way the DMT melts in and there’s less likelihood of it falling off. But with the glass slightly warmed, any DMT that hits the glass will stick (and end up not getting vaporized). So what I do now is a bit more complicated, but very effective:

I remove the copper disc from the GVG and place it on a heat-proof surface. I measure a dose on a folded piece of paper as in your photo. I gently heat the copper, then quickly pour the dose onto the disc. It melts in immediately. I then pick up the disc with the tweezers from my scale kit and put the disc back into the GVG.