As rules state currently we have the below:
A player's new salary may not be lower than the previous year's. If this is the case, the extension value will move to the previous year’s salary.
This is taken from Bush league and example how 70% rule would look:
New salary may be no less than 70% of previous salary. Therefore, a player with market value of $2.5m, current one-year deal of $5m, can have an extension for two years equal to $3.5m (0.7 * 5 = 3.5).