No, I'm talking about the total width of the cassette. Sorry, I phrased that badly I guess.

Chainline is the distance from the centerline of the bike to the centerline of the front and rear driveline components. That distance should be the same front and rear.
In the front, the center of the center ring on a triple is the measurement point. On a double, it's the middle of the chainring gap. On a single, well, you get it.
In the rear, the center of the total width of the cogset is the measurement point. Sometimes there's a cog there, sometimes there's a gap.
These measurements are more easily calculated than directly measured.
In the front, you can measure from the outside of the seat tube or down tube to the chainring or gap, and add half the width of the tube.
In the rear you can measure the cogset centerpoint relative to the inner dropout face, and subtract that from half the OLN width, and arrive at the chainline number.

If the centerpoint of the new cassette's total width is in the same place relative to the centerline of the frame as the old cassette, your chainline will stay the same, and you do not need to change your BB spindle. If it has moved inboard or outboard more than 1mm or so, you might want to adjust your chainline for that. But it will probably be ok.

Does that make more sense?