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

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"]

[tr]

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

[/tr]

[tr]

[td]

PC

[/td]

[td]

XP

[/td]

[td]

LM

[/td]

[td]

PCL

[/td]

[td]

TOT (=100%)

[/td]

[/tr]

[tr]

[td]

5

[/td]

[td]

10

[/td]

[td]

15

[/td]

[td]

70

[/td]

[td]

100

[/td]

[/tr]

[tr]

[td]

40

[/td]

[td]

50

[/td]

[td][/td]

[td]

20

[/td]

[td]

ERROR

[/td]

[/tr]

[tr]

[td][/td]

[td][/td]

[td][/td]

[td][/td]

[td]

ERROR

[/td]

[/tr]

[/TABLE]