I agree that the Fernet prevents too much sweetness but I went with 2/3 oz ea of the liqueurs. Not sure if this is something an ancient Greek king would drink but why not! I love it and am thinking of rye for next time!
I honestly thought it would be cloyingly sweet but it's not at all. The mint and bitterness from the Fernet really balances everything. It benefits from the dilution though.