Actually the first part of your post is the timer I use.

"1 glass of red = 10 mins. for the first, then 20 mins. for each subsequent".

So, on the basis that a small 'roo roast and veges takes around 90 mins. this means that 5 glasses will be consumed and the meal will be perfectly cooked.

And conveniently, 5 glasses usually = 1 bottle (a 1 bottle meal).

However when a larger, say leg of mutton, is the go then I have to utilise the cask with up to tipple I mean tripple the quantity of red timer needed.

On the odd occassion the red runs out and port has to be substituted. Under these circumstances the timer can be a few minutes out either way. 

Just keeping things nice and simple,eh. 
mikel