A "reasonable proxy" means finding something similar to the original thing, which is not completely identical but is close enough to serve as a substitute or representation. Full definition
It was the best - available reasonable proxy for the intrinsic value of assets and of the market at the time financial research began. (canadiancapitalist.com)
This is an imperfect but reasonable proxy for how index funds would have performed. (moneysense.ca)
I decided to gather the data, or reasonable proxies of it, and I ended up using the following variables to estimate the ECRI WLI: (alephblog.com)