Diamond Frames and Sizing: How Big Is Too Big?

I have come into the possession of a roadbike that was described by the previous owner as a 52cm frame, but in fact measures 54.5cm. On a diamond frame, that can be a significant difference, and I am trying to determine  - without giving in to wishful thinking - whether the bike is too big for me to ride safely. When it comes to step-through bicycles, I prefer to ride the largest frame possible, as long as I am able to hoist myself onto the saddle. My Gazelle is 57cm, my Raleigh DL-1 is 56cm, and my Bella Ciao is 54cm. But with a diamond frame, that approach is not possible, because top tube clearance is an additional factor to consider. That is why my roadbikes have 52cm frames.

To illustrate what a difference just over 2cm can make, here is the new fosterbike (right), face to face with one of my 52cm roadbikes (left). The perspective of the shots exaggerates the larger bike, but look at the space between the lugs on the head tubes to get a more accurate sense of the difference. It is considerable.

The bike is not in rideable condition, so I can't try it out. But standing over the top tube in my bare feet, there is less than 1mm of clearance between my "soft tissue" and that black cable. 

In the shoes I usually wear, the space underneath my forefinger illustrates the clearance I get - less than 1/2". The bike needs a lot of work and some component replacements, before I can actually ride it and determine for sure whether I am okay with so little clearance. And naturally, I am reluctant to do that work if in the end I won't be able to ride it. My main worry, is that when dismounting I might hit my pelvis on the top tube, since our knees tend to bend a little when we jump. But I am not sure whether in actuality that is a valid concern.

How much top tube clearance are you comfortable with? Are there standard guidelines for how much clearance is "safe"?