Preservative Recommendation for Sensitive Skin

Based on your experience with sensitive skin and your preference for the combination of Pentylene Glycol and Polyaminopropyl Biguanide, here is a recommendation for the minimum concentration for your homemade product.

You've found that 3% Pentylene Glycol and 0.4% Polyaminopropyl Biguanide works well for you without causing irritation. Pentylene Glycol is known to act as a booster for preservatives, which can allow for lower concentrations of the primary preservative.

The recommended usage rate for Eye Preserved (Polyaminopropyl Biguanide) in cosmetic formulations is typically 0.5-1.5%. While 0.4% has worked for you, it is slightly below this recommended range.

For a product made for personal use, stored in a screw-cap bottle, and applied with a cotton pad, the risk of microbial contamination is lower compared to products stored in jars. With careful and clean preparation, you can minimize the initial microbial load.

Considering the boosting effect of 3% Pentylene Glycol and your target shelf life of 3-6 months, using Polyaminopropyl Biguanide at the lower end of its recommended range, such as 0.5%, would provide a more robust preservation system compared to 0.4% for that duration, while still being a relatively low concentration that you might find non-irritating.

While 0.4% might be sufficient with excellent hygiene and storage, increasing it slightly to 0.5% offers a bit more safety margin for a 3-6 month period without significantly increasing the risk of irritation, especially since Eye Preserved is known for its gentleness.

For absolute certainty regarding preservation effectiveness, particularly if you were to produce this for others or for a longer shelf life, a laboratory challenge test would be necessary. However, for personal use, balancing the need for preservation with your skin's sensitivity, 3% Pentylene Glycol with 0.5% Eye Preserved (Polyaminopropyl Biguanide) is a reasonable approach.