If you have both fingers - or thumbs -
fully inside the foreskin so that they are
beside the glans, than you have them positioned correctly. You do need them to be "hooked" just inside the tight part so that they will not slip out and defeat the purpose of the stretch, so they
will not seem to be applying as much pressure on the narrow area as the part alongside the glans which has been stretched out by naturally frequent erections. That's OK as long as you
do feel tension on the narrow area.
(You meant to say 10 to 15
millimetres down from the tip!

)
If you use fingers rather then thumbs to stretch, you can use
more than two fingers - with as many fingers as possible packed inside the foreskin, you do not have to use your arm (or finger) muscles to hold the stretch, so it becomes much easier. And doing this will make the bundle more evenly cylindrical, focusing the stretch on the narrowest area.
We pretty much
always presume that if you could never retract before, the frænulum has not been
required to stretch, and it
will therefore need to be stretched.
If you traumatised it by forcible retraction when it was not yet retractile, it's your guess as to which part you might have injured. It
is the case that sexual excitement (including masturbation) tends to suppress pain sensation and you may only discover minor damage afterwards!
And retracting a tight foreskin is in itself a common form of masturbation.
