thanks for the help, i currently have 14 kg (28 lb) of weight with a 12.2 steel and my sac rate on the last dive was around 17. it's slowly improving i think it was around 20 at dive 12 and now i'm up to 18. 64 mins under for about 160 bar max depth was 8 m.
I suggest you to do regular buyoncy checks as you will see your need for weight probably decreasing, which will also impact on your SAC rate.
If you will always stay in a range not lower than 20m/60ft and within NDL, then a 12l would be fine. If you envisage deeper dives, then go for a 15l@232bar or 12l@300bar already now (some day you might even want to go for doubles, but that is some time ahead)
As mentioned in the previous post, do not expect your fill station to (always) provide you with a tank filled to its maximum capacity, so keep some margin, by having a higher-then-used capacity.