Just for the record. I have used volume bars in the past and switched back. Why? I find that reading the actions and intent of other players is clearer with time based bars.
On the ratio question I think its going to depend and it might be anywhere from 1:2 to 1:16 but will depend on how you trade, how the contract acts, and what the finer details give you. For HSI I use 1m 5m and 10m (was 15m but I discovered something that drove me to change to a finer read on my high timeframe). But for me the lowest tf qualifies mid tf moves and provides probable stops and targets. For Forex I use very different timeframes.
I only use volume charts. The ratio is ~1:2 to 1:4 depending on the futures. I think the ideal is 1:3.16 then your charts will be blessed.