The BLF X5/X6 driver uses a Tiny25, not Tiny85. There's a 13A, 25, 45, and 85. The 25 has a version with the same exact footprint as a 13A, but the 45 and 85 need a bigger pad to mount them properly. For code space:
13A=1K bytes, 25=2K bytes, 45=4K bytes, 85=8K bytes
Currently, Narsil is sitting at about 6K of used code space. The 25/45/85 are all code compatible, slight variations from the 13A because of additional capabilities.
The 25 has the advantage of fitting in the exact space of a 13A, while the 85 needs slightly longer pads for the pins. Alternative is to bend the pins in on the 85, which a few of us are doing to re-use existing driver board designs. Could argue it weakens the solder connections, but for use doing hand soldering reflow, it works well. For this BLF Q8 driver, of course we designed it, and there's plenty of room, so we accommodated the 85. There's more to it, though, to properly support the 25/45/85. The BLF X5/X6 driver, for example, uses a 12 uF cap instead of the standard 10 uF cap because it wasn't stable with the stock 10 uF - not the ideal "fix" though.
For the BLF Q8 driver, we've taken a new approach, as others have, like Richard, to properly support the MCU and FET+1 based design, both from experimentation and pure digital design theory, with inputs and help from Electrical Engineers experienced in this.
Lot of the public research can be found in the 25/45/85 thread: https://budgetlightforum.com/t/-/34900
Sorry to go so techie, but might help some that are interested... 