i tried several techniques and wasnt happy with most of them. Im rooting some now and will post results next month or so. If this works i probably wont tip root anymore.
Since you like Bristol im rooting Clyde now which has Bristol as a parent
https://www.pubhort.org/aps/18/v18_n3_a9.htm
It would be nice at some point to have a reliable timing and method of rooting cuttings for Rubus folks… the fig and mulberry guys have a nice network going… i think it would be great if Rubus could also have that same potential.