The amount to give for a chinese wedding dinner or even lunch is basically to cover the cost of the wedding banquet with some extra. For example if it cost $1000 for a table of 10 pax, then you should pack of minimum of $100 in order to cover the cost that the couple pay. Most people will pack a little extra like $120 instead of $100. So to decide how much to ang bao to pack for your upcoming chinese wedding dinner, make sure you check how much the couple pay for each table first.
Chinese wedding banquet unlike other wedding venue, is much easy to calculate the cost per head by using how much they charged per table. The price also does not include other external cost like rental of facilities, catering etc. All prices offered by most Chinese wedding restaurant or hotel charged based on prices per table with number of pax per table fixed.
For more information on how much per table, you may want to check out WeddingForum.Sg