Dual battery isolator- what voltage kick out?

Hi Guys

Just after some quick advice. I have a sidewinder db-120 isolator and it has served me well but i feel it may be getting tired, it is a few years old now and has had a hard life! The voltage kicks out at around 12.16v (tested this morning with multimeter) and basically kicks straight back in again. I tested this using 2 multimeters -one on continuty over the 2 isolator terminals, and the other multimeter on the main battery showing volts, read the batt voltage as the cont beep stopped. Using my headlights to draw power. When the lights were switched off, the isolator kicked back in straight away. The specs state it kicks out at 12.8v and kicks in at 13.2v and used to do this, but after a few flat batteries i decided to test it.
These results indicate that the isolator needs replacing.

I have done a search around at isolators, would like to replace it with the latest version of the sidewinder db140 but the specs seemed to have changed, the kick out voltage is now 12.4v? Do you think that is too low?
I liked the fact that the isolator kicked out at 12.8v, but am thinking 12.4 would be low, especially on cold mornings trying to start a diesel engine>
What do you guys recommend as being an ideal kick out voltage for an isolator?

i dont want to go a manual system, i want a fully automatic system.

Thanks

Doey