Staysail on a boom

I'd like to have an opinion about the right distance to have between the tack of a boomed staysail and the gooseneck of the boom.
The boat is a modern gaff cutter (Wylo II by Nick Skeates) with 3 head sails, staysail, jib and top jib. All three sails are on a pair of sheets at the moment and I am building a new boomed staysail because I want a self tacking boat when I am on my own.
The staysail dimensions are foot 12'8", luff 24' and leech 19'9".

Thanks for your attention

Thierry