Don't be silly. Nobody is asking why you're not putting 18.5 as an option, people are asking why you have random sizes that never existed in a raid setting (5 and 8) over previously existing setups (25 and 40) that has been tried and tested and people well know how they feel about it.
10, 20, 25, 40 would be the logical poll options, at least for starters. Then maybe you could add some viable options such as 5 and 30, but right now you're just mixing and matching and that doesn't make much sense.