Aw, dippy 
I had to make this decision for my old boy, aged 16 1/2 last year. Like you, he was MY dog, so the decision was mine. Add to that that I am a vet, and knew I would do it myself, so everyone deferred the decision to me.
It was very hard. I kept thinking there would be some crisis that made my mind up for me (as I have seen so many times with clients), but the reality was that he just went ever so gradually downhill. As Lizcat says, I could plan it out, and I actually brought him to the surgery, after hours- just me, DH and dd2, who wanted to be there (the others didn't)- I didn't want him to be at home, for some reason.
What I would say to you is that there is no definitive "right" time, only the time when you feel like it needs to be done. After I let oldboy go, I did feel I had left it too long, but it's far easier in retrospect, and he didn't suffer unduly.
I'd also add that THIS is the hardest bit- the making the decision bit. For months you worry about it- am I being fair, is he/ she suffering, how will I know??- and you worry that you need to make the decision, while part of you is already grieving for the dog you have, in reality, almost already lost. Once you make the decision, and it is done, you realise the reality (in terms of the actual process) wasn't as bad as you feared, and now you can concentrate on your own selfish grief, which is far easier to bear than the watching and worrying. There is a sense of relief, almost, waiting at the other side of that decision, which has no doubt been hanging over you for a while, I know it was for me. I still miss my Oldboy, but I know I did the right thing by him, and that helps. I hope whatever you decide it goes peacefully for you. x