It doesn't have to be far from full lockup to make a big difference. My Glock 19 appears to be able to fire over 1/16-inch from lockup (enough for the barrel to fall noticeably, perhaps 1/32-inch or better), but it will not fire with 1/8-inch slide retraction. Hopefully, the upgrade kit will reduce this. You can test your own, retracting the slide just slightly to see at what point it you can pull the trigger and where it gets blocked. I have not installed the upgrade yet to see what the effect is, but only the old Glocks need it -- as I understand it, all those built in the last decade or so do not, and models after the 24.
Information on the upgrade kit (and which s/ns need it) can be found at: http://www.thegunzone.com/glock/upgrade-faq.html
My writeup and photos on the kB! were posted at: http://www.thegunzone.com/glock/g19-kb.html
The Glock kB! FAQ can be found at http://www.thegunzone.com/glock/glock-kb-faq.html
-- there is a lot of info there.