public int scaleForPixelHeight (int height) {
        setPixelSizes(0, height);
        SizeMetrics fontMetrics = face.getSize().getMetrics();
        int ascent = FreeType.toInt(fontMetrics.getAscender());
        int descent = FreeType.toInt(fontMetrics.getDescender());
        return height * height / (ascent - descent);
    }
Uses ascender and descender of font to calculate real height that makes all glyphs to fit in given pixel size. Source:
http://nothings.org/stb/stb_truetype.h / stbtt_ScaleForPixelHeight
core/src/com/badlogic/gdx/graphics/g2d/freetype/FreeTypeFontGenerator.java