Nelson Sauvin is the big player with this brew. The IBUs are a touch higher than the style and can be adjusted if the extra 1% is super important. The current recipe means that it is easy to use a single 100g purchase for the batch and get a lovely hoppy IPA at the end.
Predicted losses from the grain during mash are about 2.25-2.5L and the sparge volume is based around this. Pre-boil volume was predicted to be 17L and the sparge volume was selected to give room in a 20L kettle. Essentially, use common sense with the sparge volume and, if losses are lower than expected, sparge with less water. If losses are more, then sparge with the plan, or accept a lower fermenter volume (which may be lower than the predicted 12L due to hop absorption and/or your boil losses).
Note. This should not be much of an issue if the gravities are roughly the same.