If you find
[Signal Strength 3G Gte75]) *100.00)/SUM([Signal Strength 3G Count])
Consider [Signal Strength 3G Gte75] as column2
and [Signal Strength 3G Count] as column3
Find my approach below,
Create a calculated field (Filter) to apply filter on your Column 1
Note: STR([Col1]) I've used STR() here to convert the Col1 values into string. Kindly check if this required in your original data or not
Conditional Sum: (SUM([Col2])*100)/SUM([Col3])
Once done drag above calculated fields on canvas and check. Screenshot attached for your reference.
Let us know if this help.