Engine/Transmission (1994 - 1998) Tach Increments Under 1k RPM

[ProMac1K]

Does anyone know how to read the tach in the 97's? There are two marks in between 0 and 1000, and they aren't in any equal distance fashion. The one under the 1000 would look to be 750, comparing it to the marks about 1000. Would the mark right above 0 be 250, for cranking rpm? I'm curious because I would like to know where my idle is set at. Comparing my scanner to my tach, it shows my tach being about 100 RPM higher and the scanner showed my idle in park at 864 rpm. I'm thinking about upping my idle. When cold the idle sometimes gets low enough in gear to stall the truck if you let off the throttle. But yet once it warms up, it gets right around the 1000 rpm mark.