To be fair, you'd have to optimize the 4x5 film plane flatness with a vacuum film holder. Skipping over several other pertinent variables, including...
It's tough to remember back 30 years, but I owned a Tachihara 5x4" in the early 90's and it was very good. I don't remember any obvious shortcomings,...
'Knob 18' would be (one of) the most logical to adjust the focus.
There may be a lock/brake on your knob 18, to avoid focus drift/creep OR some kind...
Is your tank made for 13x18mm? The 13x18 film will drop out of a 5x7 film holder so maybe the holder in the tank is a bit bigger to accommodate that...
This exhibition announcement came to my attention today from the Sasse Museum of Art. The accompanying appraisal of black and white photography hits...
One thing you might consider, depending on the size of your changing bag, is inserting some kind of makeshift frame or even just a suitably sized box...
I have had Apo-Nikkors with a similar slot and some filter holders that fit into the slot, but they had a moveable door to close the opening when not...