standover height significance for traditional road frame (i.e. with horizontal top tube) such as Trek Madone?

Hello

I am new to road cycling. I am male, 173cm height (~5'8" & 1/2 inches) ,
with ~83 cm inseam.

In a traditional frame (horizontal top tube) road cycle geometry such as
2005 Trek Madone, for example, what should the desired stand over clearance
be? The Trek chart says 54cm frame top tube standover is 76.9cm i.e. 6.1 cm
clearance for my 83cm inseam (~2.5 inches?). Is this frame too small? The
next size up (56cm) standover is 78.9cm i.e. 4.1cm clearance (1.5 inches)?
Better? If there a rule of thumb on this topic? Is it important?

I know from the Trek geometry chart that its top tube is 54.6cm (size=54
frame). What else should I be concerned about when trying to decide on Trek
Madone road frame size? It appears that applying one formula I know of:
Inseam*0.66 = 54.8 cm top tube length i.e. perfect for 54cm Madone frame? A
56cm Madone top tube is 56.2cm i.e. about 14mm too long for my height?

Perhaps some folks with my height and inseam can shed some light on what
size frames and stem lengths they find comfortable?

thanks in advance