The luff length is not restricted by the F18 box rules. The only things that limit the spinnaker on a F18 are:

- Hoist height: 8150mm from mast base to pulley
- Pole length: The length shall not be greater than the distance, measured in total from its attachment point from the centre of the front beam to the vertical line touching the most forward part of the hull, with the bowsprit measured when level to the deck of the catamaran, plus an additional 800mm.
- sail area.

That's it. There is no such thing as a "standard" f18 front triangle. Depending where the mast is on the platform the length of the pole may vary and as such the max luff would do the same.

In short there is no such thing as a standard F18 spi.

Some other box rule classes are measuring things differently. The F16 for example limits the hoist height and the full length of the pole. Thus the front triangle are roughly equivalent on all F16 build to the rule, and spinnies are largely interchangeable.