Ok, let's remake that calculation as I forgot that you actually get freespins if you gamble on higher spins.
If you buy 100 freespins and all start at 8 spins.
From 8->12 (50/50) 50% of bonuses lost 50 left.
From 12->16 (60/40) 40% of bonuses lost (70% of total bonuses lost) 30 left.
From 16->20 (60/40) 40% of bonuses will be 8 spins (70% of bonuses lost, 12 will be 8 spins), 18 left.
From 20->24 (60/40) 40% of bonuses will be 12 spins (70% of bonuses lost, 12 will be at 8 spins, 7,2 will be at 12 spins), 10,8 will be at 24 spins.
So if you gamble all you bonuses 70% of them will be lost with no win what so ever if you start at 8 spins with the gamble.
The 30% that will be a bonus it's 12% at 8 spins, 7,2% at 12 spins and 10,8% at 24 spins if you allways gamble all the way to 24,.
If you always stop at 20 spins then it's 18% that of your freespins will be at 20 spins, and previous % is the same.