IMO the timer should fit what you need in generel and not be a time bank that will gradually decrease and leave you in time problems every single time if the game goes long.
If your army is weak you need some time to handle hard fights and you should not be punished on time compared to the guy who runs around with 50 wyverns. Therefore 1 min is simply to little.
Time for battles should be 2-4 min. Personally I find 2 min to short but I can live with it.
I suggest 18 8 on XLs and mini nostalgia, 17 7 on 2sm4d(3) and diamond and 16 6 for balance and panic.