Question: Truncating Small Errors

Hello,

This is a problem encountered in most engineering equations. An equation is given with some variables very small that they can be ignored. Basically the mathematical equivalent of an engineering approximation.

Below is a very simple example:

Both R and r are between 10E3 and 50E3. 
gm is between 1 and 10.

So regardless of actual values the answer to 4significant figures can always be approximated by:

eqn1~ R*gm*r

Is there any way to tell MAPLE to approximate an equation such that it is only accurate to certain decimal places? I have tried using "fnormal()" with "assuming"  statements but it always requires some of the variables be assigned actual numeric values.

I was wondering if there was a way to get this approximation without having to assign values to any of the variables but only by specifying the range of values for each variable.

Thanks.