
12.5% is what you would get with a half brother to sister mating if no other relatives in common.
I have done such a close breeding twice, but would only do this in a situation such as the last where the common parent was my own bitch, who had three adult litters of healthy offspring (and was 9 years old herself by then) and the sires were imports from different continents, even so the COI was 13%.
The bitch that was mated to her half brother was also 5 years old by then with one healthy litter behind her.
I would not want such a high COI where I was doubling up on dogs I didn't know personally their health temperament longevity fertility etc.
In general, population genetic advice is that you will start to see the negative signs of inbreeding once you go over 10% and are best to try and stay below 5% if you can.