@samtim
Since the Text inside the checkbox virtually does not play any role than a label, a work around would be:
1. Set the Text label of the checkbox to " " (that is no text)
2. Adjust the real checkbox to the desired size
3. Inset a label and ensure it align with the checkbox (Picture below)
4. This should work even in a Gallery
5. formula will work same when the checkbox is selected

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