We have just installed the RTH7600 it's working fine, but I don't understand the Cool setting - for example, I have the temperature set at 68F and the cool temperature shows 82F is that right or should I lower the 82F to 71F or won't it make any difference as we do not have air conditioning.

Cool setting only works if you have air conditioning. You needed a heat only thermostat... which could have saved you a bit of money.