Sum all cells = 100%, if sum is <> than 100%, Error, if all cells blank, Sum = blank

Can't seem to get the formula to show all empty cells in the Total as ""

[TABLE="width: 249"]

[TD="colspan: 5"]PRODUCT MIX %[/TD]

PC

[/td]XP

[/td]LM

[/td]PCL

[/td]TOT (=100%)

[/td]5

[/td]10

[/td]15

[/td]70

[/td]100

[/td]40

[/td]50

[/td]20

[/td]ERROR

[/td]ERROR

[/td]

[/TABLE]