Inclusive custom fields and filters

Hi there. My client has products (industrial machinery parts) with custom fields such as 'Maximum RPM'. Values for this can range from 100, 200, 300, 600 or 800. Now, a product that has a maximum RPM of 800 can also have an RPM of just 100 (and any RPM between 0 and 800). Whereas a product with a maximum RPM of 300 cannot have a RPM of 800.

How do I set up a fieldset and filter, so that if a customer clicks 400 in the RPM field, other products with a higher RPM still appear, but products with a lesser RPM do not?

Thanks Nicolas. I've set up the custom field and filter as you suggest, but the filter is not appearing on the category listing and I can't understand why. I've done screengrabs of all the setup pages and the category listing page - can you have a quick look and see if I've done anything wrong?
Many thanks! All the best - Leo

Thanks Nicolas! Happy New Year to you! Unfortunately, none of your suggestions have worked. Could I PM you the login details for you to have a look? Many thanks!

I've solved it!

In the Filter module, under Basic Options, I hadn't selected the filters I wanted to be displayed. Whoops!

Thanks anyway!