Topical Finasteride systemic absorption?

1) Is there a certain percentage where the systematic is equivalent to 1mg orally? (Any studies on that? i.e does 0.5% reduce DHT as much as 1mg orally?) 2) Is there an optimal base to mix topical finasteride with? (As is could be mixed with minoxidil and with bases such as liposomal, ethanol and botanical bases?) 3) After having experienced side effects on oral finasteride is topical finasteride the best line of defence against hair loss? (As LLLT and PRP seem to have limited evidence?)