I don't do much floating tenon work but when I make a regular mortise and tenon, I don't worry if the mortise is a bit deeper than the tenon is long. I never measured it exactly but I'm sure I sometimes leave as much as 1/8" to 3/16". Sometimes less, of course. But I want to make *sure* my tenon doesn't bottom out. I don't think a bit of space at the bottom of a mortise affects the joint strength to any degree.
I often drill out my mortises and with a brad point bit, you leave a small hole in the center of the bottom of the drilled hole. That makes some room for excess glue, also.
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