There are a few different review scales used on this site. More might be added if/when appropriate.
Cheapskate Enjoyment Value: if you had no money to spare for anything, let alone theatre, how much would be worth your while to scrimp together to see the show. This started out as a bit of a fun measuring stick that’s not meant to represent realistic ticket prices (in fact, the value rarely goes above £10).
Time Clawback Rating: what percentage of the show’s runtime you wish you could take back because it wasn’t worth your while. Some productions are just too lengthy: there are very few things special enough to warrant 3.5 hours at the theatre watching it. Since time is money, sometimes it feels like the show should be refunding you a part of your investment in it.