Hello, adding
DHHB in the formula must be Solvent
DHHB along with the solvent. together Otherwise it will not be effective (because it is completely insoluble) using
DHHB where 5% is very high and can often make formulas sticky (or yellowish) so may be considered.
DHHB in conjunction with
Zinc Oxide partially, you can know the SPF value clearly, you need to send it for testing. The company has a lab service to measure SPF for you. If interested, please contact customer relations.