If you remember your O-level physics P*V = n*R*T. Assume temperature is vaguely constant to simplfy it to Pressure*Volume = a constant. Presume that the 7 litre bottle was down to 100 bar we have before decanting 12*300 (filled bottle) + 7*100 (emptier bottle) = 4300 ( the constant)
After decant both bottles same pressure we get P = 4300/(12+7) = 226 Bar
so all in all if you got 250 you did well. I would expect less than 226 after some heat losses.