How?!!
I don't know.
From my side the question is the opposite: I don't get why some players are never able to win the grand prize. Not even for a STS every now and then, when they like the prize a lot.
My strategy is this:
For 7 round challenges:
1st round: no sugarcube
2nd round: four sugar cubes
3rd round: six sugar cubes
For round 4, 5 and 6 I use 8 sugar cubes
I quit after level 6
For 9 or 10 round challenges:
1st round: no sugarcube
2nd round: no sugar cube if the starting percentage is 90%, otherwise two sugarcubes
3nd round: four sugar cubes
4rd round: six sugar cubes
For round 5, 6 and 7 I use 8 sugar cubes
I quit after level 7
The sugar cubes are 30 and with my strategy you would need 34 or 36. In fact I replace the sugar cubes with ads or simoleons, depending how lucky I am. Sometimes you get enough ads to increase the percentage. Sometimes I need to add simoleons.
Sometimes I fail: normally at round 1 with 85%, at round 2 with 89% or at rould 3 with 91-93%. But in general I play safe.
I use the cupcakes only for the challenges 9 or 10 rounds long and absolutely in combination with simoleons.
I use the cupcakes to buy extra sugar: 6 cupcakes for 12 extra sugar. In that challenge, I use simoleons to buy extra percentage every time that I find this possibility.
In this way I arrive to the maximum round and I make the best use of the cupcakes and simoleons. Because the royal tokens increase exponentially at every round.
Normally I get the grand prize a couple of days before that the event expires.
I hope it can help.
Your strategy seems good but I no longer have access to the ads for help, so I forced to bail out every attempt.
