I don't think that your recommendation for how to set the QoS maximum uplink kbps number is correct. According to this qos tutorial
the number should be set at 90 to 95 percent of your lowest uplink speed. Now, this is the case if that figure, that setting, is used by QoS to properly fill the pipeline with packets. It needs to know how much bandwidth it has to fill in order to properly shape the upstream traffic. Some routers automatically and dynamically figure this out, but it seems that reports are that they don't do this well, and setting it manually to 95 percent of your lowest uplink speed is best.
On the other hand, if that figure in the 2102 is only limiting the input from the ethernet port to the internet port, and leaving all the rest of the bandwidth free for the VoIP to use, then 80 percent is the number to use--or rather you'd really want to figure out how much kbps you need for your particular codex, I would think, and subtract that amount, plus a margin, from your uplink speed.
But, my experience thus far makes me suspect that lowering the QoS uplink speed does not free up more uplink bandwidth for the 2102 to use. It didn't help at all, in fact.
when I lowered the uplink number, and tested during a phone call, the uplink speed my computer saw was approximately that uplink parameter minus
a 200 or so kbps. So, I more than suspect that this paramter is TOTAL LAN to WAN throughput, including the 2102 VoIP throughput. So, you want it set to the lowest uplink speed you are likely to see, minus 5 percent or so as a safety margin.