I've written a spreadsheet to calculate approximate dive time. Follow this link to my website,
Simon Yachting - YA/RYA Skipper and go to the 'Tools' section to find the Dive Time Calculator.
Plug in your SAC rate, tank size, tank pressure and depth and it'll tell you a theoretical bottom time.
It also includes a SAC rate calculator from dive history. Plug in your start pressure, end pressure, tank size, dive time and depth and it will give you your SAC rate. If you don't have this history from your last dive, plug in 15 l/min (I hear that's average).
7 litre @ 136 bar should give you 22.67 mins @ 18m
3 litre @ 300 bar should give you 21.43 mins @ 18m
Essentially the same...
2x 10 litre @ 200 bar should give you 95.24 mins @ 18m
15 litre @ 300 bar should give you 71 mins @ 18m
-- That's until absolutely empty, not allowing safety and ascent time -- This also assumes constant SAC rate, which may not be the case with different configurations and different conditions. All in all, it's only a mathematical model, not real life - treat it only as an estimating guide.