I'll echo Jb's comment about going slow in trimming the fillet. I usually chop the fillet on my Morso about 1/8" longer than my measured inside opening. Then I sand the fillet with my hand disc sander in very minute increments until I get a perfect fit.
It is EXTREMELY easy to over sand or trim as there is so little wood in a fillet compared to a frame and the fit needs to be exact. There is much more tolerance in making a frame than in fitting a fillet. Don't count on any ruler as a measuring device...fit by feel.
When you order fillets chopped, the vendor will usually ship them slightly oversize so that you can trim them to fit the mat. I can't imagine why anyone would order them chopped if you have to trim them anyway unless you are using something like an expensive water guilded fillet and only need a foot or two and don't wish to have exstra in stock.
One thing I do different is that I join the fillet with wood glue and put the assembled fillet into the mat. Cutting another reverse bevel mat for the back is a nice touch, but not really needed if you put the proper thickness of panels in and bumps the cost also.
I am interested in trying the Fillet Master. I have my system down pretty good, but anything to speed production is a plus.
Congratulations on your first fillet! I'm sure you'll do fine. A fillet really can complete a package and set your work off from other "custom framers" and the BB's. You'll find that your work will begin advertising your shop when you do higher end work too.
Now...fabric wrapped mats and fillets...mmm-mmm.
Dave Makielski