Vertically align \framed with itemized list using ConTeXt

Vertically align \framed with itemized list using ConTeXt

Problem
The numbers in the following itemized list are not aligned with the text:

Code
It is possible to adjust the vertical alignment using the offset command:
% Itemized bullet list
\definesymbol[StyleBulletSymbolNumeric][{%
\offset[y=0.2\baselineskip]{%
\StyleBulletFramed[
width=1em,
]{\small\small\currentitemnumber}
}
}]
However, the calculation of either y=0.2\baselineskip or y=0.2\lineheight
does not quite vertically align the number and its background to the text.
Question
How would you vertically align the number of an itemized list such that:
the number is vertically aligned within the frame; and
the frame is vertically aligned to the itemized text?
Ideas
I have tried:
location=middle - produces a frame that is much too low
command=\vcenter - no discernible effect
\offset[y=0.15\baselineskip]{% - comes closest:

In the above image, the numbered item should be 2px higher with respect to
the \framed object, while the y value for the offset is almost perfect (I
am concerned that varying the font and its size will cause the alignment
to be off; that is, it's a fragile calculation that should use a constant
rather than a magic number of 0.15).
Example
A complete example that shows the problem:
\setupcolors[state=start]
% Defines the frame style for bullets. This replaces the leader dots in the
% ToC as well as provides the starting point for all squares.
\defineframed[StyleBulletFramed][
frame=off,
width=0.5em,
height=\framedparameter{width},
background=color,
backgroundcolor=red,
]
% Defines the bullet as a symbol, which can be used with bullet lists.
\definesymbol[StyleBulletSymbol]
[{\StyleBulletFramed{\strut}}]
% Defines the bullet as a symbol, which can be used with numbered lists.
% Twice small to ensure the number fits inside the bullet.
\definesymbol[StyleBulletSymbolNumeric][{%
\offset[y=0.15\baselineskip]{%
\StyleBulletFramed[
width=1em,
%location=middle,
command=\vcenter
]{\small\small\currentitemnumber}
}
}]
\define[1]\instruction{\startitem #1. \stopitem}
\defineitemgroup[Instructions]
\setupitemgroup[Instructions][each][joinedup]
\setupitemgroup[Instructions][
symbol=StyleBulletSymbolNumeric,
location=middle,
]
\starttext
\startbodymatter
\startInstructions
\instruction{Set up the video camera's tripod at the hallway door}
\instruction{Start the video recorder}
\instruction{Make sure the path to the hallway door is unobstructed}
\instruction{Ensure both laces on your running shoes are tied tight}
\instruction{Check that nobody else is in the room}
\instruction{Don safety goggles}
\instruction{Using metal serving tongs, hold the sodium over the water}
\instruction{Drop the sodium into the water}
\instruction{Run to the hallway}
\instruction{Prepare to call the fire department}
\instruction{Upload video to YouTube}
\stopInstructions
\stopbodymatter
\stoptext
References
http://wiki.contextgarden.net/Enumerations#.5Cdefineitemgroup
Vertically aligning list bullet when using \infer
ConTeXt: vertical spacing