You can uses this equation, but you have to follow some rules:
* because of the small channel effect, the width of the most has to be at least 2.5 -3 * min. width (look at design rules)
* length has to be at least 2.5-3 * min. length
The best is to let L constant. A good practical value is in your case ~ 0.6um. So you only differ the width, that works good for hand calculation.
But its better to use a spreadsheet like excel or gnumeric. The best is to write a small script which invokes your simulator-engine and puts out the bias of your most. Using a loop, you know how to achieve a target value (W/L), and you can also get a estimation of process variation. That's what a designer really needs.
In case you need low noise performance, low offset,... - you have to adjust this method, multiply W and L by the same factor.
btw: are you using the tsmc, umc process?