Ran a little fun calculation. This is under the assumption that 50/7 drops 5x2 guaranteed souls with the 3x2 souls being 50% so needs to be taken with a grain of salt.

Assuming the average is 13 souls from 50/7, i would need 30 SC to get 1400 souls. This counts in for stamina. Charisma needed is within what i can get from 11 days.

Assuming i only run 80/9 with its 15, i would need 35 SC from stamina alone and then another 0-5SC depending on how much i level to restore charisma. At my level it takes about 3 days of running the 40 stamina desert map and the 80/9 doesn't even give half of that exp while costing the double charisma. So i'd say i will level 2-3 times, meaning a total of 37SC needed.

On the contrary, at the worst possible, only getting 10 souls, i would need 44-48 SC depending on leveling and at 16 souls that would be 21.

So with this i do wonder - can anyone confirm the drop rates from 50/7? Have you missed one of the 5x2 souls on a full clear that was not your first?