Great way to mount. Don't like seeing them on good running well maintained A-4's. I do like seeing them on engines with issues for diagnostics.
I too have used a spark plug for checking spark however it can be difficult to see the "jump of the spark". I recommend removing the ground lug from the spark plug. First this gives a larger gap to jump and it is far easier to see!!l You will also be able to "audibly" hear the "snap" as well as see the spark. The bluer the better. A spark with a yellowish flash is not good.
FYI, the hi-voltage ignition (the spark) needs to jump across a gap which is no big deal but when you add compression to the "chamber" this compression also offers up a great deal of resistance which is why you need a blue snappy spark!
Dave Neptune