Couple of additional thoughts, I use my Schneider 180 mm makro for shots of orchids with 4x5.
The dimensions that I start out with using this lens is 360mm of extension and about 180mm distance from the subject and then move from there (bellows extension and the whole tripod and camera) to get what I want. At this setting, my exposure is 1.5 to 2 stops over metered to compensate for bellows extension. If one is to err on exposure, err on the side of slight overexposure - 1/2 to 2/3 stop for some punch to the finished image.
Longer lenses will increase the working distance from lens to subject, but, the depth of field decreases and the images tend to flatten.
Buster 6 x 6, I have a good friend here in southern California, Paul Tuskes, who uses a setup similar to yours and does it very well. I think his photos were used recently to illustrate a book on phalaenopsis.
The American Orchid Society has a guide book on photographing orchids, recently updated.
Their guidelines to me for photographing awards is one flower, head on, dominating the frame. Very boring, so I tend to meet their requirements, but include additional flowers to add interest to the frame and show the habit of the inflorescense.