There is an article that have described what you might just need. The idea is to put negative sign in front of those measures which you want to be in percentage. This allows you to custom format those numbers.
If you are wanting to set the number of decimal places then the best thing I have found to do is convert the values to string and then use the Left function to display only the part you want. In order for this to work in any totals or grand totals, you will also have to convert back to a number when doing the calculations. So if your value is 23.12345 and you want two decimal places, you need to say str([value],5). You may also want to look at the sixth character and determine whether or not the fifth character needs to be increased by 1.
Thanks for the tip - much appreciated.
I have to say it would be a very useful addition to have some sort of function that allows you to do this, and therefore could be used in a calculation.
Thanks Sharad - this article exactly describes my predicament!
It's a very nifty work around, and I have used it to get the desired outcomes.
Thanks again for the link.