@randydj
You can also try to shorten the far-red end of day treatment from 15 to 10mins untill the stretch is gone. You need between 2000 and 4000μMol(not per second) to switch phytochrome far-red. The more you use the more stretch you'll see. A far-red trigger with 5w at the wall and 44% efficiency needs only to run for 5 minutes to throw out 4000μMol.
(5w x 0,44% = 2,2PAR/w x 6μMol/J(QER for far-red) = 13,2μMol/s/m² x 300sec(or 5mins) = 3960μMol within 5 minutes)
I don't know how powerful your far-red trigger is but 15 minutes could be more than needed...
Yes, the flower cycle shortening effect is stronger the more far-red you use but that is only true up to a certain range. Above 4000μMol the shortening effect get's more and more negligible but the effect on stretch is still there up to the point you see SAS symtomes(shadeavoidingsyndrome).
Personally I begin the flower session always with only 2500μMol and increase to 4000-5000 when the stretch is done.
If I use too much far-red from the beginning my tops later fall all over the place 'cause the skinny stems can not hold the weight. I need to use plant jojos or bambus rods then to avoid this.
If you know the used diodes and the driver of your far-red light we could figure out how much time it actual needs to switch your plants into night mode.