Hi,
This site is obviously full of well paid expats. 40k a month should allow you to live well - there will be prudence but no skrimping and no scrounging. The only Cartier watches you will be buying is at Tsim Sha Shui.
I read somewhere - I think on Batgung that the easiest conversion for someone coming from the UK (not sure if you qualify) is that your monthly HK salary is the same numerically as a yearly UK salary.
i.e On 40,000 HK a month you can maintain the same standard of living on in HK as you could in the UK with GBP40,000 a year.
Hope this helps. BTW I am on a similar wage and can - with some prudence - save about $10,000 a month - and I have two kids.