Well you claim trading from 20 to 40 and from 40 to 20 is the same ROE. I would say half.
Further you claim percentages don't matter, only absolute numbers. So I'm curiuous how you can make $50 from shorting from a price of 40. (No leverage, 1 stock and 1 trade).
*and no joke the 40 is measured in e.g. pound
