The component parts have the potential of resulting in a characterful drink. Recipe as written is way too Montenegro heavy for me. I went 2:1 tequila to amaro second time and it wasn't quite right, even after doubling the agave and adding 2 drops of saline. I want to make this work...
So it being a Saturday night and us not needing to be in church early Sunday morning, we did a side by side tasting: firstly, original recipe simply substituting Nardini for the Montenegro; secondly, same with Braulio; and thirdly, substituting Bianco for the Reposado, Nonino Quintessentia for the Montenegro and lemon for the lime. The Nardini variant was our favourite but the Nonino version was lovely too, with lemon or lime.
We suspect that the problem may not be the volume of the Amaro but the particular Amaro chosen, Montenegro, which for us is a workhorse and not a racehorse (at 45ml, a racehorse is what we think might be required here). We might try Nardini, Braulio and Nonino Quintessentia to cover a few bases...maybe even Averna. We salute your perseverance.