What is definition of Tallit Gadol falling off

At what point are you required to make a new bracha on yout tallit if it slips partially off your shoulders
See 97:4 OC


According th the Shulchan Aruch, any time it falls off. According to the Shla, never (since we don’t wear a Tallit all day. Same as Teffilin)
Because if this, the Rabbi recommends making a Tnai that if the Bracha is only until its taken off. Then we make a new Bracha every time.

It’s hard to quantity falling off when it slips halfway… if you’re not sure, safek brachot lehakel