Hi Count
To budget for your wedding dinner, start with a worst case scenario. For eg: Assume your guests will be giving an $80 angpow, and cost is $100. You shld be prepared to spend at least $20 per person. If you have 40 full tables, that will come up to $8,000.
If you also have your guest list ready, you might want to skim through and do an estimate from there. I had a few guests who gave $60 ang pows. But there were a good number who were surprisingly generous and would give $120 - $150 ang pows. Our peers are most likely to give $80 - $100, with good friends giving $100 - $120. Closer relatives will tend to pack more. So sometimes you'll be able to cover from the generous ones.
In your overall costs, you will also have to take note of extra expenditure (eg: wine, extra beer, etc).
For my wedding, we also pooled the ang pow from our church ceremony and tea ceremony. And so we managed to cover from there. In fact, i think in the end, we actually had some leftover cash as pocket money for our honeymoon!
So if you actually set aside some money to spend, you might find yourselves having a little 'bonus' in the end : )