No Pete, you don't format formulise the decimals. The {Field}*10 example is actually an entire formula. Create a formula with just that line in it, and use that to return your percentage.
Out of interest, are you querying a database field to give you your percentage, or are you using a formula to return the percentage? If it's the latter, post it up and let's see what you got. I think we should be able to set it straight so you don't need to create additional formulas.
Naith