As far forward as it will go......
Basically this is right if you have any sort of Winder, and it is generally right for pretty much everything else.
BUT, Severns are different apparently,
and conventional wisdom (ie, Pete Bond) says that they prefer their mast foot a bit further back. The Severn I sailed went pretty quick and suffered from weather helm, and I reckon that it would have
benefitted from a forward step. But I didn't try it, so I can't say for sure that it would have gone as quick if I had. Further forward = less weather helm, but what's it gonna do to your boatspeed.
Should improve it of course, but will it ?
Assuming you've got the adjustable step, I'd stick the mast as far forward as poss, secure in the knowledge you can always get it back to how it was
using the adjuster pins ('cos you measured it before you changed it, right). Easy to say of course - but every time you move the mast foot, you have to adjust the rake and the pre-bend to compensate, so
you would have to be pretty happy about setting up the mast to do this.