Since you asked... (as taken from the patch test protocol, cited above)
Originally Posted by kumquat
2.2. Test articles
The amount of test article applied should be expressed as the quantity of chemical per unit area of the skin, lg/cm2, and as a percentage. A vehicle that contains ethanol (EtOH) must be used to dilute the test article. The preferred vehicle of RIFM is 75% diethyl phthalate (DEP)/25% EtOH. In addition to the test article, the test subject should also be patched with the vehicle control and a saline control. It has been well documented that the vehicle in which an allergen is presented to the skin has an eﬀect on its skin-sensitizing potency (Kligman, 1966; Lalko et al., 2004; Stotts and Ely, 1977). The DEP and EtOH combination in use by RIFM was selected because the majority of fragrance materials are soluble in DEP and EtOH, it is representative of the matrix often used in commercial products, and the 3:1 DEP/EtOH ratio was selected since on rare occasions increasing levels of EtOH can induce sensitization in humans (Stotts and Ely, 1977).
2.3. Experimental design
2.3.1. Induction phase
The quantity of test article applied per test patch is 0.3 ml or 0.3 g. The test articles are dispensed onto 25 mm Hilltop Chamber patches (Hill Top Research, Miamiville, OH) and the patches are applied to normal skin between the left scapula and the spinal mid-line. Hill Top Chamber patches are composed of a ﬂexible molded plastic chamber with a double rim that ﬁts close to the skin and the chamber is lined with a nonwoven Webril pad. The patch is held in place with semiocclusive tape. Fresh patches of test article are prepared daily. Following application of test article to the patch, the patch must be applied to the skin a minimum of 15 min after preparation but not longer than 40 min after preparation. Sample preparation and volatilization times should be documented...
...Test article patch applications to the same patch site are generally made on Monday, Wednesday, and Friday for three consecutive weeks...
...A minimum of nine Induction patches are required in order to satisfactorily complete the Induction phase of the study.
It goes on. There are apparently Challenge and Re-challenge phases for those sensitized in the Induction phase. Seems rather exhaustive to me. As it should be. And .3ml is far from overload imo.