Well let me explain the "ish" part for ya. So I think this exact seating die has a slight amount of slop in it and will be off a couple thousandths each time potentially, I really should get a better one to test and see if it just me or what. Now for the "how to read" part, I will try to as un condescending as possible as I understand not everyone helps rebuild rear differentials and stuff at work like I currently do so others may not be well versed in dial calipers...
ALSO, my caliper is a standard and metric combo unit so I will be focusing on the inch part for this exercise. The bottom of the ruler is inches and the black needle and black numbers are inch increments.
Here a marked up pic of the dial caliper.
View attachment 1184910
If you can read my autistic diagram, I labeled that by reading the flat metal ruler (like a tape measure for you carpenter types) you get 1.2 something inches but can't tell exactly what since that part isn't fine enough.
That's where the dial comes in.
View attachment 1184911
So by looking at the dial from the top green line I drew that is "0" as in if this was a 1in long COAL the black needle would be at the top and the slide ruler would just barely be at the 1in mark. But since this is a 1.25ish COAL really more of a 1.255 COAL the line is reading just past the .25 mark but not at the .26 mark yet. As indicated by the second green line I drew.
Hope this helps not just you, but others as well.