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Your thread title refers to a mergefield, but there is no evidence of one in the body of your post. For example, if there were a mergefield, I'd expect to see:
{= 0.1 * {= {MERGEFIELD Field} / 10}}
or:
{= 0.1 * {= «Field» / 10}}
That said, it seems you could get the same results with:
{=0.01*{MERGEFIELD Field}}
or:
{=0.01*«Field»}
As for the record whose field doesn't 'work' in the formula, you might check that the number is all the field contains.
Note: The field brace pairs (i.e. '{ }') for the above examples are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues. Likewise, the chevrons (i.e. '« »') are part of the actual mergefields - which you can insert from the 'Insert Merge Field' dropdown (i.e. you can't type or copy & paste them from this message, either).