Ijust bought some strings to do this also should I keep the upper Brass ringand use the lower rubber donut to support the lower knot or add an additional brass ring to support the lower knot???
No I would mark the string where the brass nock is then take it off and tie the top of the loop to where the brass nock is as for the "rubber donut"
just slide it down the the serving near the cam or carefully cut it off.