As I said, I never used that informative Annex G of EN 1591-1. Now I understand why it has no correspondent in EN 13445-3- because that Annex G belong to EN 1591-1 seems to be beyond the engineering common sense… It is not sound engineering presenting incomplete information about notation used and I don't understand this approach under European Norm!

You are right, in G.3.4.5 "Forces in subsequent conditions", they present the fluid pressure force F_Q(I*) - that ends the spacer contact- as:
F_Q(I*) = F_SR(0) / η_SR(I) (G.23)
but there is no explanation on η_SR(I).

Probably there is a way to recover the meaning of η_SR(I) if the information in EN 1591-1 Annex G is enough to reconstruct step-by-step the flanges rotations and flexibility parameters related to each status of this nonlinear evolution (due to contact or non-contact with the spacer). It may be interesting for someone who has enough time for it, but is not my case.
In addition, I consider that what really is missing in EN 1591-1 Annex G is they don’t consider any additional (external) load in method- now I understand better why... but now the utility of this special part of EN 1591-1 appears as questionable for a lot of practical applications.

My best regards.