The tin hat target is easiest, because the centre line (bottom of the tin hat) can be lined up onto your sight blade with a teeeny line of white visible. If no tin hat, just staple a piece of white paper across the bottom half of your black - voila.
If your eyes are like mine, you will get a better depth of field with a No 4 (peep sight) than a No 1 (vee sight) because of the pinhole effect. This can be enhanced by making the aperture (pinhole) a bit smaller. There are many methods, the one I use is to get some matt black paint and a fine brush, and line the rear sight hole with a layer of paint. When dry, repeat, so the hole gets smaller. Keep doing this until you see the view through the peephole becomes a tiny bit darker. Then that's optimal.
If using a No 1 (SMLE) with a vee sight, you will do better with either a slight correcting lens so you can focus on the foresight (ignore the target and the vee sight - they'll be slighly blurred, but you can't have it all) - or a pinhole (orthoptic) in front of your eye. See Knobloch Schiessbrillen Shooting glasses etc.
Finally, you should ensure your sights are blackened (use a zippo lighter to apply soot, or puff on a little Birchwood Casey Sight Black).