I am looking to make a formula that checks a range of cells in a row
for those completed with content and those blank,
but creates a percentage for those complete,
however it needs to exclude the value 'No'.

If the row in question is A1:R1, for example, this might help:

=(COLUMN($R$1)-COUNTBLANK($A$1:$R$1)-COUNTIF($A$1:$R$1,"No")) / COLUMN($R$1)

Hope this helps getting started.