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You already can use percentages. To add 10%, you would write something like =n*1.1.
But I don’t think it would be very useful, to tell you the truth. Because in my experience, percentage adjustments do not make sense in metrics. Think of an O and a Q with a tail that extends beyond the RSB. If you add 10% to the RSB, the RSB of the O would grow, while the RSB of the Q would decrease. And that is just one example.
@Mekka: I just think about the problem like in web-layouts using px and em.
You could be right, but it is the responsibility of the designer to use those opportunities or not?!
For me, I try to think more in percentage than in fixed units : ).
Sure. As explained above, you can already do percentages, so it already is your responsibility. Feel free to go ahead, but don’t say you were not warned. I have yet to see a sensible use case (or good reasoning, for that matter) for percentage adjustments, or, at least in Latin typography, one case where it did not create more problems than it solved in the first place.