JMW 9" Tone arm

Can anyone help with advice on the best way to set the tonearm height on a 9" JMW arm. The manual suggests using a 5 x 7 card is this the only way, does it matter if it is 5 x 7 or 6 x 4 or....If this is the only way why is there not one included? it all seems a bit hit and miss, so is it that important?
You must be referring to the technique of using index card to adjust the hight of
the arm. The size of the card is not important. What is important are the parallel
lines on the card. So either size will work but the larger size will be easier.
You might be interested on recent thread concerning arm height changing sound after you have set up.
Using the card is only used to get the arm level with the platter. This is a starting point. After you have your arm level and your catridge alligned and the tracking weight set, listen for a bit. Then lower the arm a bit, re-check everything and listen. If it's sounding better adjust down a bit more and repeat. If it sounded worse, raise the arm a bit, listen, see what you think (hear). Repeat.

This will take some trial and error, but that is how you learn and get the most from your rig.

Also- Get a Mint LP Protractor for excellent cartidge set-up. I'm sure you'll find this tool recommended plenty here.