In hindsight, my dummy order has an FX estimate of 1.167 and,...

In hindsight, my dummy order has an FX estimate of 1.167 and, like I said, the current rate is 1.176, so the dummy order seems to be estimating that I would pay LESS in AUD than what I should!

The Reserve Bank website should SGD / AUD as being 1.1771 so e*trade's estimate seems to have a fairly tight spread of 0.1%.

It really depends on what the trade executes at, but the estimate seems reasonable as far as FX conversion charges are concerned.

I see that some other brokers (like Saxo, I believe) don't convert to AUD for every trade. They only convert when repatriating money back to Australia (I guess).

This is good if one plans on trading multiple times - they just keep funds in SGD on their broker's account and trade in and out whenever they like without FX fees each time.