@thomas.zdon - I don’t believe this would be possible as you cannot write a formula that accesses previous row values. This post confirms this: Repeating records with increment dates based on previous row value
You can set a field to increment and you could randomly choose 0.5 or -0.5 but you wouldn’t know where the value had got too as all the previous values would be unknown to that row.
You could built a dataset with the random 0.5 or -0.5, download it and then use Excel to do running total.