I use a ruler and X-Acto. I buy a dispenser that holds 50 blades, and change every other print or so.
I too like the "floating mount", where there is space between the outer edge of the print and the inner edge of the window mat, with extra space at the bottom for a signature.
Here's what I've come up with. A standard 11x14 mat for 8x10 artwork has a window that is 7-1/2" x 9-1/2". For a floating mount, I leave 1/4" around the two sides and the top, and 3/8" at the bottom, for the signature. For 11x14 prints, I add 1/8" to all sides, but that's just me.
So, I had a window mat cut with an opening that is 7" x 8-7/8", and the opening is 1/8" above center in the vertical plane. I use this mat to overlay the print and mark it for trimming. Then, I put this mat on top of an 11x14 mount board, line them up and fasten them together with clips. I can then drop the trimmed print and mounting tissue into the opening, put a shot bag on it, and tack it down.
Seems to work pretty well.