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Now to your question. Line height of 1.2 at body level is pretty small. 1.5 or higher is preferable. What do you mean by having double spacing in your table cell text? That 1.2 line height will cascade to every other element so it will be applied to your TD text also.
Line-height: 15px Will put exactly 15px between each line, so if you change the font-size, this won’t change (and will break on any font above say… 15px (which will look mega cramped))
line-height: 1.2 and line-height: 120% are essentially the same, If your font-size is 15px your line height will be 18px (15 * 1.2) but if you changed your font-size to 20px for example, the line height would be 24px (20 * 1.2). 120% is also the same (20*120%=24)
You could also set your line heights with ems if you wanted which would scale with the font-size but seeing as an em is the height of the letter ‘m’ (hence em’s) its probably about 60% of the font-size depending on the font family and just makes working things out a little harder
I personally use unit-less measurements for line-height eg 1.2 and personal preference is 1.3 or 1.5 for body copy.