Try to make it more simple. We are not actually trying to make you know you have grown 66.7%, or a certain number. A lot, a little? Normally we know whether we've grown a lot. Sometimes you see one Parami is a weaker one or one is stronger, or maybe there is just a little difference. Then we are not actually letting doubt get in the way. Normally for all of them, if you are practicing properly, you have grown some, right?
I can understand this one because Steve had to help me with this one some time ago. He said, "just say, yes, you know, it's okay to say, yes". Sometimes if we have trouble to say "yes" it means that we have self-doubt. So there we would need to work on practicing some confidence inspiring reflections to lessen the tendency to say "yes but..." or not acknowledge our growth at all.
If you are thinking, '"I don't know how I have grown because I could have grown more in that time and what about that time, etc." Then usually you are lost into doubt meditation and it's helpful to just have a few words, "yes" or "a lot", etc. Sometimes when I come to some Paramis I can say, "Yes, that's a good one." Maybe that's enough for you then, you know you have grown a lot. I know where my strong Paramis are, I know the ones that are weaker and usually the strong Paramis will help to bring the others up if we don't get too lost in the doubt meditation when we are trying to do a confidence inspiring reflection of the Paramis.
Allow yourself to have some joy with it. That's why I think the next question is for, "How do I feel about that?" Yes, allow yourself to feel some happiness with some growth even if it's only a little. Sometimes we get too greedy and we want a lot. So even if it's a little, "Yes, I have grown a little", it's better than not at all.
And sometimes another person may be able to see the growth much more than we can. So if you have doubts about it, perhaps go to talk to a kind, spiritual friend, a Kalyanamitta, who knows you very well and say, "I'm having trouble with this particular Parami. From your observation, have I grown a little or a lot?" Hopefully we will be able to take joy in our own growth and not need the encouragement of others. But that's okay to have a good spiritual friend. Sometimes a good spiritual friend will believe in us when we don't believe in ourselves, which gives us the confidence to develop until we can appreciate ourselves.