To abandon the confirmation dialog mark out or delete the lines
// confirm theh two channels;
var theCheck = confirm("please confirm\nchannel a is "+channelA.name+"\nchannel b is "+channelB.name);
if (theCheck == true) {
and the corresponding closing bracket.
Does reducing percentage make the result less accurate?
Naturally, but possibly not in the sense you mean.
The Selection from the Path is intersected with B and A anyway, so it will not extend beyond those.