When do Mercury/Venus reach greatest elevation at sunset/twilight for a given location?
You can take a look at this table - it's in Dutch, but the dates and numbers in the table shouldn't be too hard to understand.  The first date is that of the greatest elongation (GE), the second (halfway the columns) that of the best visibility (BV).  You should ignore the lines with red dates - they're unfavourable appearances, and the code may find a date far from that of the GE.  The table shows that the difference between the moment of GE and that of the BV is typically a few days, although it can be about a week. (You can click on the first date for a page which includes a sky map of the appearance at constant Sun altitude (-6&deg; I believe)).

Venus is a different story, because it is usually visible for months.  As you may know, in the northern hemisphere morning appearences of the inner planets are unfavourable in spring and favourable in autumn, and for evening appearances this is the other way around.  The table shows that Venus had its GE (west/morning) on 8 June 2001, but was best visible on 22 August.  In the mean time, the planet had moved somewhat closer to the Sun, but the effect of moving from spring towards autumn was clearly stronger (click the date link for the weird map that goes with this).  Hence, for Venus the difference can be several months.

Of course, these are just examples for one location (52&deg;N).  I think the process may be too complex to be described by a single, simple equation.