Where the SC of your storm sail is will depend on how you want to go thru the storm.
Most of the time a small part of the
genoa furled out makes an excellent downwind storm sail. Especially when supported on the
spinnaker pole.
Upwind, a
storm jib alone, also one on the inner stay, may blow you off too far. Hence a storm main, a trysail, or some alt method is required.
I think only proper (designed, not converted) will go upwind well under a storm
jib on an inner stay.
b.