I was always under the impression that you can't give putts conditionally?
But I also don't understand the purpose of it. I only give putts, if I'm 100% sure my opponent makes them. It doesn't matter whether he gives me my putt or not, because if it's 100% for him, then why should my putt have any bearing on that?
I wouldn't think it in the spirit of the game if I started to calculate percentages and then offered a good-good only, if the percentages are on my side. Let's say I think he makes his 95% of the time and I make mine 90% of the time, but he thinks he makes his only 80% of the time, whereas I make mine 85% of the time, then we would both agree on good-good and we both think we pulled a fast one. But to me that has nothing to do with golf.
Ulrich