Unfortunately I have never found a reliable GUI for configuring such a thing. You'd have to use command line to alter the settings. It's a very ugly and hairy route.
This isn't what you're asking but here's a script I use on my own Acer Aspire. Perhaps you can use it:
#!/bin/sh
sleep 5
xinput set-int-prop "SynPS/2 Synaptics TouchPad" "Synaptics Two-Finger Pressure" 32 10
xinput set-int-prop "SynPS/2 Synaptics TouchPad" "Synaptics Two-Finger Width" 32 8
xinput set-int-prop "SynPS/2 Synaptics TouchPad" "Two-Finger Scrolling" 8 1
xinput set-int-prop "SynPS/2 Synaptics TouchPad" "Synaptics Two-Finger Scrolling" 8 1 1
xinput set-prop "SynPS/2 Synaptics TouchPad" "Synaptics Edge Scrolling" 0 0 0
If you make a .sh file, mark it as executable and set it to run on startup then you will have disabled edge scrolling and instead have fairly decent two-finger scrolling.
I do not know of any method to change the threshold, unfortunately. I have looked around many times for such settings. Xorg is severely limited in these areas, unfortunately.
xinput list
output from terminal.