black double monk strap for formal with captoe or without
captoe or without captoe better ?
These days most businesses have "business casual" dress codes and so are not the most formal outfits.
Certainly double monks are less formal than an oxford is but in most business situations they'd be perfectly fine. Technically without toe cap is more formal than with toe cap - with or without double monks - but personally I prefer toe caps.
For formals, well it depends just how formal it is. I've been to some places where you wouldnt be allowed in with double monks and been to others where well polished double monks would make you the best dressed in the room (footwear at least)
Strictly formal wise, you should be wearing a black oxford (not even a derby or blucher). I am referring to black and white balls, or those that you wear tuxedo where a brown shoe would be considered unacceptable.
Single strap is usually considered more formal than double straps, but I think it's usually acceptable to wear a plain double monk with or without captoe for formal occasions. But then, who really will notice except you? The ladies will likely notice, but they wouldn't be able to tell what a captoe is.