One Cell Blank or Formula

I want to create a user defined function (if possible) that acts as follows:

-Is either a formula or blank where I can select a range (includes total) -I am trying to either put in a total for a group(70%), or define each of the constituents of the group. If I input the total as 70%, I want each of the constituents to total 70%, however, if I enter the constituent values, then I want the total to add them up. In the example below,the constituents don't add to 70%, but should (based on their prior %). Also can't add to >100%

-Item 1 50.0%

-Item 2 1.0%

-Item 3 0.0%

-Item 4 60.0%

-Item 5 0.0%

-Item 6 0.0%

-Item 7 0.0%

-Item 8 5.0%

Total 70.0%