Hi manitobamacro,
Straight to critique on your first macro shot, brave, i like it ..
The colours are good, the focus is ok but its being affected by the low light. Make your shutter speed longer ( I hope youre using a tripod at 1/15 sec at 300mm otherwise
) or consider external light. I would also consider reframing/recropping the shot, its too small a slice to appreciate the subject even if it was inteded as more asbtract type shot. The sharpness of the shot will improve with light as long as your focusing is correct which I believe you did get right in your shot - middle stamen.
I would also recommend a close up filter over an extension tube on a 300mm lens, bit too much weight on an extension tube for my liking but the choice is yours. I'd recommend a raynox 250 or 150 (250 for more magnification, 150 for less). Nisi also makes a close up filter but I prefer Raynox as its a clip-on system that can be quickly mounted and removed versus Nisi's filter which is a regular threaded lens filter.
Let me know if you have any questions.