Longbow: height of nocking point

Hi,

I was browsing the FITA intermediate coaching manual for longbow (does anyone know who wrote this, btw?) and it said that a high nocking point could result in erratic arrow releases. Any tips on judging the best height for a nocking point?

Also, I understand that some archers prefer to nock their arrow below the nocking point rather than above it (assuming you have a single point and not a pair). Are there advantages to this?

Thanks