I have created a spreadsheet for you to a) use and b) to learn from.
It is an Automated spreadsheet (as they should be) which calculates the number of minutes in a working week or month and calculates the average time per email giving Daily, Weekly and Monthly Outputs. It takes into account Public Holidays (or for time off). You can use the Output to create Graphs etc to visually display the Output.
It also allows you to calculate a Part Month average.
I have displayed it as it was CONSTRUCTED and as it would be USED.
The As Used worksheet is Protected and the only Inputs that can be done are in the Green Boxes (also the Saturday and Sunday boxes but you will need to Unhide the Validation List to include these and then to add 2 more columns titled Is Saturday? and Is Sunday? with the appropriate If Statement.
To unprotect the sheet go to Tools - Protection - Unprotect. There is no password so leave this blank.
All the workings are still there, the columns are just Hidden. To Unhide them, highlight the columns to the left and right of the hidden columns, click on Format - Columns - Unhide. To hide them again, highlight the columns that you want hidden, click on Format - Columns - Hide.
The LOGIC used (as in Functions) may seem complex but if you read the Descriptions in the first row you should be able to work out what and why it was done that way. Click on a cell to see what Function was used where.
You said that your spreadsheet was becoming a real mess, well I have created a monster for you (but not a mess).
I have uploaded the file to here:
Hope this gives you the push to really start using Excel.