The lowest line, which on the slider, lines up with a line on the staff, is the reference point. The first sight reads 25 points. The next sight is 27 points, the next 28. If the slider were raised one more, it would read 29. If the slider were raised so that the outside two marks were lined up with the next lines on the staff, the reading would be 30 points.
Different shooters prefer different front inserts for different conditions.
For black silhouettes and bullseye targets, many shooters like apertures. Frank Zikka will supply different sizes if requested, but know that almost no one can shoot with less than an .085 insert, because there simply isn't enough light. Traditionally, apertures are used only from a resting position. For offhand shooting and for shooting at white or orange targets, many choices are available. Either a pinhead, small enough to center in the target, or a crosswire modified from a crosshair by cutting off the top vertical are preferred by some of the top shooters.