Doing a "drill-down" calculation

I'm trying to build a "drill-down" system in FileMaker and I've run into a wall.

Table "Fonts" has a portal pointing to table "Keywords", which in turn has a field called "Keyword Count" that, via a self-assigned relationship, counts how many times a given keyword is used. When I look at my layout in "Fonts" I see the keyword (e.g., "industrial") followed by a number; the number indicates how many fonts have this keyword attached to it. Any given font can have any number of keywords. That part works fine.

I wrote a script that, when you click on a keyword in the "Fonts" layout, omits any fonts that don't have that keyword attached. That part also works fine.

What I need to have happen is that number needs to dynamically change with the criteria. For example, let's say there are 200 fonts, of which 100 fonts have the keyword "industrial" and out of those 20 also have the keyword "deco". I first click "industrial" which reduces my found set to 100. I then click "deco" which reduces my found set to 20 - but the number next to "industrial" still reads 100. I need it to reflect not the total number of fonts with that keyword, but rather the total number of fonts with that keyword in the found set.

I've been experimenting with all manner of calculations and it never quite works the way I need it to. What's the best way to get this done?