If your main is not class legal but less than 1.05 times the stock area you have to take an MN adjustment (0.995 * D-PN). If it is more than 1.05 times the stock area then you take an ML adjustment (0.98 * D-PN). A non-class legal sail is always assumed to be faster regardless of its size.

There is no official adjustment for non-class legal jib of same sail area or less.

A boat with a lower portsmouth number is faster, so you want a high number.

If your boat is class legal for Hobie 18SX other than the sails, then I think you can make a case that you should start out with the Hobie 18SX number. If it is not, then you do need to start out with the Hobie 18 number and take an additional hit for the spinn. This makes a big difference.

So it looks like if you start as a Hobie 18 you take an MN and SP adjustment. If you start as a Hobie 18SX you take an MN and JU adjustment. The latter will get you a higher number. You should double-check the sail area of the main though - "same sail area or less" is defined as less than 105% of the stock area.