I've found a good link, I think it's very suitable for beginners if one finds interest. If I found this link months ago, then it must shorten my learning a lot. Don't know who is the author, but it's definitely useful.

電算機隱藏的祕密

Yes this page has been around for a very long time, explaining the basics of pari mutuel system.

Maybe the author copied some of the concepts from US, and tried to apply to HK, some of the formulas presented are wrong or in a strang format.

E.g. the very first formula on the page, it should be simply Odds=Si/S, instead of [(Si-1)/S]+1. It's written in such strange format because the concept is translated from US style odds, where the odds is expressed as profit, instead of total return including the cost.

As a result of using US style odds, the expectation formula in the middle of the page is wrong if applied to HK odds.