Or is it 70:30? 
As in, if you're an ok parent for 80/70% of the time, you have a bit of leeway to be a bit crap?
I'm beating myself up for a rubbish evening last night - absolutely lost it with 11 year old dd for getting paint on her new school uniform, leaving me with a painty, messy plastic bag for me to deal with (I still have a blue arm this morning!) and not getting on with her homework.
The evening continued on a downward spiral, dealing with 15 year old dd who has a limited diet due to ARFID and refused to eat spag bol. I just can't feed this child 
I feel crap at shouting at dd3 - she is ASD and gets easily stressed. I'm her mum, I should be helping and supporting her so what do I do - lose my shit with her 