% Gain in a portfolio after adjusting for cash movements

I have tried to get a % gain calculation up based on;
1. The amount in cash and shares at the start of the month,
2. Adjust for withdrawals and additions of cash during the month,
3. Determine the % movement in the portfolio after adjusting for the cash movement.

I get the following graphs as % gains / losses from the spreadsheet, although it is probably not correct to just sum the % moves.



The % gains of my portfolio on a monthly basis are shown below;


It's relatively easy for me to work this out now that I am balancing my cashbook and timing issues over time will be pretty irrelevant.

I better go and check to see if I have to take any action next week.

regards
Stevo