tickler taper

I have cut the slot for the tickler and at the moment it is 9mm for almost its whole length except at the exit to the nut socket where it is currently 7mm.  Is this correct; does the tip of the tickler taper down, or does it remain a constant thickness? Also, should I be cutting a teeny bit of slack into the slot to cope with expansion/contraction of wood and metal with temperature and humidity changes?? If it makes any difference, I am not intending to use a spring.
Thanks,
Andrew

Geezer here, concerning tickler/triggers. Usually I make my ticklers from 3/8 in. (9mm) square mild steel bar.  I usually flatten the forward end a bit where the trigger bolt goes thru, to give me more steel on both sides of the bolt-package, and it usually ends up a bit less than 3/8 from the top bend forward, and narrower still (perhaps 7 mm) at the distal end (fits into the sear-keyway)  The tickler's tail is usually either rat-tailed, or flattened into a spoon.  Period medieval bows, which were usually  stronger than mine often have thicker trigger/tickler thru the center... as much as a half-inch (12mm) square, tapered at the head and tail.  As for slack, I don't intentionally put slack into the trigger passage, but usually there's plenty of space.  If the trigger is too loose in the passage, I'll shim it out with leather pads.
As for trigger springs: I suspect plenty of medieval crossbows were entirely springless. I have made a few that way, but generally my customers expect a trigger spring, so I include them.  Geezer.

Thanks Geezer, that's exactly what I needed to know! 
Andrew

If I was making bows over @ 200 lb. draw I would move up toe 1/2 inch steel for triggers... just to clear the pivot-bolt if for nothing else, but that would require more tapering, front and rear.  As it is, @ 200 lb. is about the limit for my stringing jig.  My journeywoman, (known as 'lightly' here) built her jig substantially stronger, so she can make more powerful bows.
Over the years, I have discovered customers don't really enjoy bows over 200 lb. anyway.  They need a spanning device, everything has to be built stronger, they break bolts and bowstrings.  That's another reason I've never increased the strength of the spanning press.  Bows you can span by hand... say 125 to 150 lb. are fun to shoot.  Over 200 lb. gets to be work, you need a better target, a safer place to shoot, better bolts, heavier bowstrings.... it gets to be no fun.  Geezer.

I think this one is heading for about 150 lb, might be needing a belt hook; I'm hoping to avoid a stringing jig by carving stringer grooves into the knocks.