Your task is complicated a little due to the fact that your green arc is not a simple little path arc that goes from left to right (or vice versa). It is actually a closed shape that defines the outside of the green shape. So it goes from left to right around the outer radius, then a little straight bit to the inner radius, and another arc back to the start.
It is possible to fill a green shape like that, in a meter like way, using a mask or a clipping path. However it is much simpler to just convert your green meter shape to a simple path that goes from left to right in one direction, and has a thick width.
If we do that, your SVG becomes, something like:
<div align="center">
<div class="circleDraw">
<svg width="116" height="100">
<g transform="translate(58,58)">
<! the green path converted to a simple one directional stroke -->
<path d="M -39.8,33.4 A 52,52, 0, 1, 1, 39.8, 33.4"
style="fill: none; stroke-width: 12; stroke: rgb(100, 150, 100);">
</path>
</g>
</svg>
</div>
</div>
Now that we've done that, we can use a really easy method to produce your black meter line.
What we do is make an exact duplicate of the green line (except black this time of course). Then we apply a dash array to the line that makes it on for 50% of the length and off for 50% of the length.
The length of our arc is 236 units. So we set the dash array to:
stroke-dasharray: 118 118
<div align="center">
<div class="circleDraw">
<svg width="116" height="100">
<g transform="translate(58,58)" style="fill: none; stroke-width: 12;">
<! the green path -->
<path d="M -39.8,33.4 A 52,52, 0, 1, 1, 39.8, 33.4"
style="stroke: rgb(100, 150, 100);">
</path>
<!-- the black path -->
<path d="M -39.8,33.4 A 52,52, 0, 1, 1, 39.8, 33.4"
style="stroke: black; stroke-dasharray: 118 118;">
</path>
</g>
</svg>
</div>
</div>
If you need to change the meter to read something other than 50%, then you need to adjust the value of the stroke-dasharray
. Just make sure the two values add up to (at least) 236.
This other question shows how to update the meter with Javascript if that's what you ultimately want to do.