When you are not selling 100% then its the same thing whether you give stakeback and split profit or just split winnings. In this example if you win 100k and give the backer his money back which is 8,000x0.7=5600 and you keep your 8000x0.3=2400 then the profit is 100000-5600-2400=92000. The backer gets 70% which is 92,000x0.7=64,400. In total he gets 64,400+5,600=70,000 which is the 70% of the total winnings. It is clearly the same thing