That's one fantasy, whatever captures your imagination. I think more about what an executioner might do on the spot, with no prior measurements, and with the goals of maximizing the victim's time on the cross and her freedom of motion so that she could struggle and put on a good show for the crowd. Just rules of thumb that an executioner might follow.
I have a couple of problems with stretching the arms out along the patibulum as far as 60 degrees or more: First, the pull on the shoulders on one end and against the nail in the wrist on the other becomes very high - maybe enough to pull a shoulder out of joint sooner or later and disable a victim so she will die fairly quickly.
And second, it limits her range of vertical motion. At 60 degrees, she would hang with her shoulders about a forearm-length below the point at which her wrists were nailed, and she could rise to a point at which her shoulders were about a forearm-length above the nails. Her legs are longer than her arms, so her motion is limited by the way her wrists are nailed.
I chose to have my victims' arms outspread at 30 degrees from the vertical instead. This minimizes the pull on her shoulders and the nails in her wrists, and it maximizes her range of motion on the cross so that it's only limited by the length of her legs. And it's easy for an executioner to measure; all he has to do is place her wrists at about the width of her outstretched elbows and the geometry will take care of the rest. The victim herself becomes the measuring stick.
I always have my executioners nail her feet with the victim hanging by her nailed wrists, just pushing her feet up until her soles are flat and heels up close to her butt, and nail them there so that her humiliation is maximized and she has almost the full length of her legs to raise herself.
Placing a cornu is trickier; I want the cornu or sedile to mount in an augered or chiseled hole in the cross, but boring a hole with a victim's body hanging in the way is clumsy. I wondered if there could be a way to locate a hole where a cornu could be mounted to fit any size victim?
Perhaps Roman executioners had a rule of thumb for that too. The best I could come up with was to use the proportions artists use for drawing the human body, all in terms of head lengths. Using an analysis based on that, I came up with some figures which Balbus tells his new slave, Gundericus, in The Serpent's Eye:
“Ja, Romans have a system for everything! But the cornu, how Antius knew how high to place that? He measured her?”
“Thirty-two inches from the level of the wrists to the point of the cornu. It works for any victim except the really tall ones. If you use a sedile, then it’s just like a cornu with no horn. Add another four inches and it’s thirty-six inches to the top edge of the sedile. The same hole in the stipes will work for either one."