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You can set the power of the flash manually, using either a flash meter or just by referencing the histogram of the test shot(s) on your camera, or you can use the TTL flash metering of the camera. If your subject has an even range of tones, either will work well. If the subject is mostly dark or mostly light, the TTL meter can be fooled, so in that case setting it yourself will often work better.
As for color, the best course is often to match your flash's color to the ambient color using gels, and then set a color balance. But if the subject's color is the most important factor and you don't mind the visible lights in the scene going warm then it's fine to set a color balance from a flash-lit neutral target. In your case, the light fixtures may not even be visible in a macro shot, and the flash will easily over-power the ambient light, so the color of only the flash is what you'll balance.
Setting the color balance should be done with the flash power setting that will be used, because changing the power can cause a color shift in the flash, although with Speedlights the shift is minimal compared to most studio strobes.
Yes, the target should be as close to the subject location as possible.