Why does this change whether or not you can estimate it that way? The tick size in the US is at least $0.01 for reasonably priced names so it's completely unrea

Why does this change whether or not you can estimate it that way? The tick size in the US is at least $0.01 for reasonably priced names so it's completely unrealistic to expect an HFT to make money on every trade. When people talk about making $0.0001 per share that's their ex-ante expectation. It accounts for the fact that you're not going to make money on every trade.