1) The income, and assets are different. This is not a fair definition.
For one team (the one who can pay much more than this 2/3 value) it will mean tanking until the end of the season and then push harder than anybody else.
For another team (a very poor one), it will mean use all their money from start, without investing at anything else (Arena for example).
2) This rule had not been set at the NBA for elimintating tanking.
The idea is to have a better level of competativenss, where the difference between the teams at the same league is limited (by the Cap-Floor's rules), and is being pushed forward for having better teams, by this rule.
3) Setting this rule upon a specific team's revenue is also not enough.
As the difference between the two teams above is big, one could save much more money during this process and buy a much better players at the end of this period, "minutes" (close to the closing TL window for the playoff) before the playoff.
4) My suggestion will make users think carefully during season (like a real manager do), and take action right away before season starts.
They will not be able to tank without paying a big price for that.
They will be able to improve their team upon season progress, but under the following posibilities (limitations);
a) Replacing players from their roster but keeping their salary cap.
b) Breaking the salary cap, but under the 10% limitation I've brought as an example.
c) Breaking the salary cap above this limit, and paying the price.
This is similar (in a sense) like the price a team in the NBA pay for breaking the Cap.