30L is for 90 glasses. 60L is for 180 glasses.
It should be enough with the facts that not many people drink beer nowaday.
To be safe, 1 table need around 1.5 bottles of wine.
It also depend on your banquet, is it a weekday dinner? lunch? or friday and saturday dinner? Festive month? (CNY, Xmas, mid-autmn)
People tend to drink less on weekday and lunch.
Drink more when there is other festival.
Most importantly, look at your crowd, older? younger?
From the last banquet I went in swissotel, we notice that beer was mostly drank by elderly. Younger generation prefer red wine.
Not to forget, how many drivers around cause they can't drink (much).