Ask HN: How do you do pre-orders the right way?

I have a HW product I am working on. Initially I will only have a small number of units available but I expect I will probably sell a few more. To accommodate this I would like to offer a pre-order page on the website. Some things I am trying to figure out:

- What percentage of the unit price should I charge for the pre-order if any at all or the full amount? - What kind of conversion should I expect from pre-orders that did not commit to any or partial payment? - What kind of wait is acceptable? I estimate it will take around 2-4 weeks to produce more products but it's highly dependent on part availability which could extend this to 2-3 months. - Should I not do any pre-order but just sell as soon as I have stock? I don't want to run too much stock because I have no idea on the demand.

Has anyone done something like this before and can you give me some pointers? I highly appreciate any feedback, thank you.