My wife and I are almost exactly the same height, and we find that the most comfortable standing position seems to be when she is up on her toes (probably makes her about 2 inches taller than me). Some considerations may be how far she plans to bend over, because height affects your angle during penetration and angles are everything!
My best suggestion to you is to take something that can be added in increments but still is hard-packed enough to hold shape (I would recommend newspaper because it stacks well and is cheap) and wrap it together securely so it doesn't slide. Make "trial" runs making the stack thicker or thinner until you get the right height. Then "Measure twice/cut once" and presto, you have your preferred platform.
P.S. we tried the heels for the height difference but balance made it too dangerous so make sure the platform (or whatever) has plenty of stability all the way around for all the rough rockin' & rollin'.

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