There are different perspectives from which to calculate the gain, but the way I think it should be done is with respect to the risk you've assumed in the original position, which the simplistic calculation doesn't factor in.
There's a good explanation about calculating the return from a short sale at Investopedia. Here's the part that I consider most relevant:
When calculating the return of a short sale, you need to compare the amount the trader gets to keep to the initial amount of the liability.
Had the trade in our example turned against you, you (as the short
seller) would owe not only the initial proceeds amount but also the
excess amount, and this would come out of your pocket.
Refer to the source link for the full explanation.
As you can see from the other answers and comments, it is a more complex a Q&A than it may first appear. I subsequently found this interesting paper which discusses the difficulty of rate of return with respect to short sales and other atypical trades:
The problem causing this almost uniform omission of a percentage
return on short sales, options (especially writing), and futures, it
may be speculated, is that the nigh-well universal and conventional
definition of rate of return involving an initial cash outflow
followed by a later cash inflow does not appear to fit these
investment situations. None of the investment finance texts nor
general finance texts, undergraduate or graduate, have formally or
explicitly shown how to resolve this predicament or how to justify the
calculations they actually use.