yes, its always based on the slices in the pie. 0.5% is the smallest a slice can be and it was mentioned that 20p was the smallest amount that a pie can handle. so whatever your smallest slice is would be determining the minimum amount you need to deposit to trigger autoinvesting. the best way to reduce the minimum is to just increase the size of your smallest slice, alternatively move that smallest slice into its own pie with a few other really small ones so they all have a larger portion.
this will apply for both deposits into the pie and dividend reinvesting inside the pie. once the amount is passed all funds in the pie are then used at once.