The relevant IRC rule is 26.3.4:
26.3.4 RRS 50.4 shall not apply.
A headsail is defined as a sail tacked down forward of the foremost mast with half width (measured as a headsail) not exceeding 75% of LP. Any other sail set forward of the foremost mast is a spinnaker and shall have half width (measured as a spinnaker) greater than 75% of foot.
The rule applies in the order it is written. Therefore:

If the sail satisfies the definition of a headsail (ie. HHW not exceeding 75% of LP)
then it rates as a HEADSAIL.

If the sail does NOT satisfy the definition of a headsail then it rates as a SPINNAKER, and then the SHW must be at least 75% of SF.
As an example, for the following sail:
Measured as a headsail:
LL 15.90
LP 8.47
HHW 6.00 (71% of LP)
Measured as a spinnaker:
SLU 15.90
SLE 14.90
SF 8.91
SHW 6.70 (75.1% of SF)
When measured as a headsail, HHW is less than 75% of LP. This sail therefore rates as a HEADSAIL. It is then irrelevant whether it complies with the spinnaker definition or not.
If you have any queries on this, please get in touch with Mike Urwin or Jenny Howells at the RORC Rating Office, tel 01590 677030.