The formula you need is
Overall Ratio = Spur Gear / (Pinion * Internal Ratio)
So (and my math is rusty), transposing that, I think you get
Internal Ratio = (Spur Gear / Overall Ratio) / Pinion
Failing that, I have an excel file that you can play with to your heart's content.
If you pm me your email, I'll send it to you.
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