@cybmole,
Oh dear, your 'dropcap conundrum' is making me smile... I went through exactly this kind of faffing about a couple of years ago when I first realised that epubs could display them.
I twiddled the values of line-height, margin-top and font-size until it looked 'just right' on the PC - then realised it didn't look the same on my reader (Sony)... so round the loop again... finally it looked good on the reader...
... then I discovered you could change fonts on the reader.
Horrors! the dropcap looked terrible in a different font, the vertical placement was all wrong again.
I wish there was a happy ending but, for me, I decided enough was enough. The ability to switch fonts on a whim is non-negotiable for me so I settled for a set of parameters which looked 'borderline OK-ish' for all the fonts I use.
I think the variation in up/down appearance caused by which font is being used may be something to do with certain capital letters whose glyph descends below the baseline in that particular font. There needs to be room to accommodate those descenders. If you're testing a particular font, try the 'J' and 'Q' as dropcaps.
For myself, I'd almost lost the will to live by this stage. If you find the magic formula, do share... but try not to go mad in the process