I know. I merely meant to point out the general relationship but obviously there isn't a 'number' that explains if iv changes x amount then time decay changes y amount. It's just a generalisation - if more time to expiry then premium will be higher than if less time to expiry. If vol goes up then premium will be higher than if vol goes down. Nothing more than that. Sorry if I didn't express it better earlier, I just thought it was fairly obvious. db
I always found it a very good analogy and I use it all the time as a mental shortcut. In fact, in the P/L spreadsheet I use I always have the number of days to make up for a change in IV at hand. Can be positive or negative. Also the theta/vega ratio can be of use. Ursa..