I personally recommend making a stencil from the original photo of the part with the halved Bermuda onions (rose madder color ones), and dust on some detail for the concentric circles.
Doesn't have to be heavy handed and can be appropriate colors to set it in context. and you can blur it here and there and even paint a little onto it, just so it doesn't look out of context and mechanical.
Just a thought. Save you going blind trying to eyeball it.
"Not a bit is wasted and the best is yet to come. . ." -- remembered from a dream