this anwers my questions.
Since I never saw a "persistent-contrail" in my life the first 50 years of my life my guess is that that time we had only homogeneous freezing only at RHi of 145% to 165% or higher.
At -40 Celsius 148.3 RHi is the same as 100% RH with regards to water. 145% RHi would be about 98% RH with regards to water. No wonder I never saw a "persistent-contrail" in Europe prior to 9/11 in 2001.
Now lets look at the heterogeneous nucleation of 110% RHi or more. I think this is related to cloud seeding and silver iodide.
110% RHi is 10% than the frost point in the yellow table higher up. Will add a new table for this in a day or two.
|supersaturations in excess of
40-50% with respect to ice =
Use the figures in the table on the left and add 2- 5- or 10% for RH values with regards to water.