Ok so there are lots of different designs that supposedly have
different strong points. Suppose I just want to go for maximum dB. How
do I achieve that? Does anyone have a site that explains what
parameters to use to achieve that?

And can anyone explain this. When I ran the DL6WU Antenna Design
Program and fed in my parameters (433.92MHz, 5mm thick elements,
plastic boom) it gave me a set of lengths and spacings and a supposed
dB of 12.1. But it had the DE and D1 3.4mm apart from centre to centre
meaning that with 5mm rod they were actually touching! I separated
them a bit and fed those lengths into Yagi Analysis 3.4 and it gave me
a dB of 4.9.