I completely agree with
@NicoleC1971 : correction doses are an art.
Unfortunately they vary too.
They may vary at different times of the day and they may vary at different BG readings and they vary per person.
The different times of the day part tends to follow similar patterns and times to your BG dose varying: unless you have a pump or a meter that calculates dose based on a timetable of insulin to carb ratios, most people will have an average correction rate. I
think 1 unit for 3mmol/l is a common starting point for someone who has left their honeymoon period... at least it was mine.
The different BG readings is a little easier although it varies per person. Basically, as our BG rises, we are likely to become a bit insulin resistant so need more to correct. For example, for me, when my BG is over 15mmol/l, I need twice as much insulin to correct 1mmol/l.
Finally, the per person bit: the calculation you have quoted is guidance. It will depend on things like how many carbs you eat (compared to many on this forum, I eat a lot of carbs but my correction dose does not take this into consideration) and some indecipherable things which some higher being (like a research scientist) may understand but I certainly don't.
When trying to work out what works for you (or your friend), I would always underestimate, test, top up if necessary and keep your hypo treatment to hand.
I tend to correct every 2 hours when I am very high and use the rule of thumb that my body has used half the insulin I last injected. Like all of this calculation the "insulin on board" (IOB) is an approximation and, in this case, it is conservative.