Henderson will seize chance when his number gets called
Studying maths in his spare time, Ulster's Iain Henderson won't need a text book to tell him that Ireland's second row is summed up by an uncomplicated formula - three into two won't go.
So far in this Six Nations championship, Henderson has started, missed out through injury and been on the bench, three differing scenarios as he and Donnacha Ryan vie for the spot alongside Devin Toner in Joe Schmidt's engine room.
Against Scotland, it was Ryan who was missing with Henderson scoring his fourth international try in the defeat at Murrayfield.
Henderson suffered a minor hamstring strain before the trip to Italy but returned off the bench against France just over a week ago.
It seems likely he will do so again against Wales in Cardiff on Friday, with Ryan's international renaissance, and a style more suited to playing from the start, meaning it is expected the Munsterman will be in the four jersey come kick-off under the lights of the Principality Stadium.
Should the pattern indeed hold, it could be seen as a frustrating championship for Henderson.
When Ireland were last in Cardiff, they were preparing for a World Cup quarter-final against Argentina with Henderson in the side.
While the Paul O'Connell era wasn't yet officially over, he had played his last game in green the week prior against France, the man who had been a number 8 during his school days at Belfast Royal Academy seemed the surest bet to take on the mantle as Ireland's most influential lock.
That was thanks to his eye-catching displays earlier in the tournament when the now 25-year-old treated some fellow internationals like rag dolls.
It's all anyone has been expecting since.
In a poor Ireland performance against Scotland, he didn't have the same impact as in those memorable autumn days of 2015 but to listen to some you would think he had been anonymous.
It was unfair, but with expectation still so great, Henderson admits he has to be his own harshest critic.
"I've just got to keep plugging away and when you do get an opportunity off the bench, you give everything you can, put in your big hits and carry the ball as much as possible," he said at Carton House yesterday.
"That's all you can do. If you start looking further afield and start bettering yourself without bettering the team first, it won't work out well and the team won't go well.
"It is difficult to take emotion out of playing the game because I love playing for Ulster and Ireland.
"But you have to have some sort of cut-throat instinct when looking at your performance, analysing yourself, making sure of what you have done.
"A lot of it is put into stats for us. I don't mind going through it and figuring out whether I have bettered myself from my previous performance.
"A lot of it does come down to being brutally honest with yourself and having the stubbornness almost to push yourself through, looking at how badly you did things.
"Realistically, it is the bad things you have to concentrate on to get better.
"You can flick through to all your tackles made, your good ones, your carries, your line-out takes.
"It does take a bit of effort to sit down and analyse yourself. It has to be done.
"If you don't correct the things you do wrong, you won't get better.
"You learn that from top level players, watching them look at their mistakes to better themselves."
The idea of watching the greats closely is not new for Henderson, especially in an area of the game expected to be key come Friday night.
While, given that Henderson's early career saw him keener to play blindside than lock, an easy comparison to make was always with former Ulster, Ireland and Lions hero Stephen Ferris.
However, it is another former Kingspan favourite that Henderson cites as key to his appreciation of the intricacies come line-out time.
Wales hooker Ken Owens has not had a single dart stolen yet during this championship and, while Toner has gathered more than twice the number of throws as the next Ireland player on the list, the Scarlets man has a varied list of potential targets.
Out of touch is an area that Wales will certainly look to make hay.
"Johann Muller for me, towards the end of his time at Ulster, he got in the huddle and we got more clarity, an extra five seconds to think about the call," said Henderson.
"Justin Tipuric is a massive threat at the front for Wales and Sam Warburton is an athletic guy, I'm sure he is easy to lift and he gets up very fast.
"We have analysed what they do in certain areas of the field, whether it is a four-man or five-man, six or seven maybe further up the pitch. They have a good variety, not just in the championship but in November.
"It's good to pick out their trends and patterns."
Even for the mathematical mind of Henderson, it's a tough equation but one Ireland will need to crack.