Gain / Loss on account data

HI all, I'm having a problem writing a formula to calculate the day to day increase / decrease in value on some accounts.

For instance, the top part of the spreadsheet shows the value of the account on a particular day, the bottom part shows the increase / decrease on that day.
The increase / decrease on the account should be calculated from the initial day and show and increase / decrease if the balance changes on any particular day.

I had some luck with the following formula, that another forum member kindly provided =IF(E8="","",E8-$B8), this calculates the change in balance correctly if the balance does not change on the previous day to the balance being calculated, or there are no changes in balance since the first day.

For instance on days 2 and 3 for account number 4 the difference between the days would be a balance of 600-100, i.e. 500. The formula in its present state (cell E17, highlighted in yellow) calculates it as -200 (using 300 – 600) because it has taken the starting balance and not accounted for the previous days balance.

I would be most appreciative of any insight you can provide on the above and the attached.
Thanks for your help, Steve.