Not sure why you couldn't find the rule as it would have been created when you sourced load_players.sql.
If it isn't there for whatever reason, this will create it with the value set to true:
Code:
INSERT INTO `rule_values` VALUES ('1', 'Character:SoDClientUseSoDHPManaEnd', 'true', 'Setting this to true will allow SoD clients to use the SoD HP/Mana/End formulas and previous clients will use the old formulas');
This will change the value to true if the rule already exists:
Code:
UPDATE `rule_values` SET `rule_value` = "true" WHERE `rule_name` = "Character:SoDClientUseSoDHPManaEnd";
If you go past level 85 using Underfoot the client doesn't know what your base HP/Mana should be, so it defaults to 1 for both and all you will see on the client is the difference between that and what your gear adds. You can use #mystats to see the real values.