DaveG
Airbush Analyst
now I'm dumber than I was earlier
hahahahah - you are just overthinking it. As stated earlier, for use on the HP-C Plus, you are better off using the nozzle cap from an Iwata HP-BH. The original part number is I 140-7. If you order from an online retailer, ask them to remove the cap from the package and confirm that it will fit the HP-C+ body before shipping.
If getting a .2 nozzle cap is too difficult, put your .3 nozzle back in, and try the brush like that. It may just surprise you.