Is there any specific algorithm relationship between Letter spacing and font size in InDesign?

Hi @dublove5CFE, the issue here is converting tracking units (which I assume you are using to space out the A in the second line) into ruler units. Tracking is measured in em units which relate to the font size. The conversion factor is, 1000 / font size. Some fonts have differing sized em squares but Indesign seems to calibrate them all to 1000 units in terms of the UI and scripting API. You can see how it works in this script, that attempts to solve your problem.

/**
 * Letterspace Chars To Align Tab.
 * Script expects to have a text selection that includes paragraphs share a tab stop,
 * and will adjust tracking of characters before the tab character so that the last
 * characters in each paragraph align.
 * So that the script can be run multiple times, it resets tracking to 0 each time.
 * @author m1b
 * @discussion https://community.adobe.com/t5/indesign-discussions/is-there-any-specific-algorithm-relationship-between-letter-spacing-and-font-size-in-indesign/m-p/13861581
 */

function main() {

    app.scriptPreferences.measurementUnit = MeasurementUnits.POINTS;

    const emSquare = 1000;

    var doc = app.activeDocument,
        text = doc.selection[0];

    if (
        text == undefined
        || typeof text.findGrep !== 'function'
    ) {
        alert('Please select some text and run script again.');
        return;
    }

    // find paragraphs with tabs
    app.findGrepPreferences = NothingEnum.NOTHING;
    app.findGrepPreferences.findWhat = '^[^\\t]+\\t';
    var found = text.findGrep();

    // store the horizontal position
    // of the right-most tab
    var positions = [],
        maxPosition = -Infinity;

    for (var i = 0; i < found.length; i++) {
        found[i].tracking = 0;
        positions[i] = found[i].insertionPoints[found[i].insertionPoints.length - 2].horizontalOffset;
        if (positions[i] > maxPosition)
            maxPosition = positions[i];
    }

    // track out the initial characters
    for (var i = 0; i < found.length; i++) {

        if (positions[i] == maxPosition)
            continue;

        var chars = found[i].characters.itemByRange(0, found[i].characters.length - 3).characters;

        for (var j = 0; j < chars.length; j++)
            chars[j].tracking = ((maxPosition - positions[i]) / chars.length) * (emSquare / chars[j].pointSize);

    }

}; // end main

app.doScript(main, ScriptLanguage.JAVASCRIPT, undefined, UndoModes.ENTIRE_SCRIPT, "Space Chars Before Tab");

Edit 2023-06-15: reset tracking to zero time.

Edit 2023-06-16: now correctly handles no characters before tab.

In the script you can see the conversion into em units here: <ruler distance> * (emSquare / chars[j].pointSize). Where emSquare is 1000.

Note that for convenience I altered your example so that every paragraph has a tab character to tell the script where to align. The script will only work if there is a tab in every selected paragraph—it will try to align the text before the tab. Here is what it does when I run it:

Let me know if that helps.

- Mark

What EXACTLY are you trying to achieve? 

A,B,C,D,N,a,m,e - each one has a DIFFERENT width chosen by the font designer. But they scale reasonably exactly with the font size - if you go from 12 point to 24 point the letter spacing (for each letter, for that specific font and weight and style) is exactly double.

Letter spacing (kerning values) are set by the type designer and generally vary withthe combination of glyphs. For consistent spacing no matter waht the text might be you would need to use a mono-spaced font like Courier, or create your own.

I suspect this might work better if set up as a table. You could then use full justification in your paragraph styles to completely fill the width of each cell regardless of the number of characters.