The chance of getting spirit plans is 5% for 3rd monster and 10% for 4th so it is definitely not a sure thing.
On the Wiki it says that the overall chances of getting a Spirit plan is 1/12 for 700+ scores and 1/2 for 800+ scores. Is that incorrect?
Well based on my experience, I seem to get plans from 700+ about 1/10 times or so and from 800+ about 1/4 time or so. Very rough guestimates.
I may be wrong on the estimates. That is what I was told they are. In my experience it is actually too high. In the 8 months I have been playing I have only received 5 spirit plan drops. I just assumed that I was unlucky. Maybe you are really lucky.
Plans really do drop a lot when you get 800 scores.
You get one once in about every 5th 800 score.