In this, there is a comparison with the man he replaced as skipper, Ross Taylor. Taylor's test average is 42.84; as leader in 13 tests that jumped to 49.85. McCullum's rise is not so significant, but it is there.
He averages 35.72 in his 74th test. This is his fourth in charge, and in that admittedly small sample, he is going at 37.5.
He opened the batting in the two tests in South Africa, through circumstance as much as anything. Peter Fulton was earmarked for the job but was back home when the first test began with a knee injury.
McCullum tried to play the anchor role and it fitted as well as OJ Simpson's glove. Now he's in his ideal spot, at No 6, where it is easier to assess the situation.
Where Hamish Rutherford talks of his fondness for walking out with the score 0 for 0, McCullum seems to like to know the score, so to speak.
His performances taken across the six limited-overs internationals and two test innings against England, are 10, 74, 26 (T20s), 69 not out, 74, 79 (ODIs), 74 and 69. Apart from a weakness in the 70s, he appears to be relishing the situation in personal batting terms. Throw in a willingness to push a range of buttons in the field with his settings, trying to make something happen, and there are encouraging signs.
The way he assumed the captaincy will always have an unpleasant whiff about it; that said, McCullum has been assertive since returning from South Africa.
Yesterday, he walked out to a packed Basin Reserve at 85 for four, Kane Williamson having chipped a catch back to Broad, when looking good.
Dean Brownlie went lbw in the next over to a good delivery from Jimmy Anderson as the ball nipped about - as it hadn't done to a noticeable degree over the first two days - and the acid was immediately on McCullum. He found a solid ally in the resolute BJ Watling and they put on 100 by contrasting methods. Watling played the junior role to the extent that his part in the century stand was 22.
McCullum won't die wondering and no matter the situation, he has an offensive mindset. He was well beaten early by Broad but seems to have an ability to move on quickly from those moments.
Give him a chance to cut or pull and he's up for it. Broad was planted over the boundary at square leg yesterday and he gave Steven Finn something to think about at one point yesterday with a rush of runs.
There are times he needs to temper the urge but that's his game and it's got him to a point where only four batsmen have scored more test runs for New Zealand than his 4323.
Just when you started thinking a seventh test hundred was on the horizon, McCullum was caught at second slip. At that point, New Zealand were 77 shy of the follow on.
Tim Southee came and went, witlessly hooking a bouncer straight to fine leg, but Bruce Martin demonstrated he should be batting a place higher with a second conscientious hand in as many innings.
In South Africa, Watling was New Zealand's most technically adept batsman and his application was rewarded with a fourth test 50 yesterday and further evidence that the wicketkeeping-No 7 job is his for at least the medium term.
Broad blew the last two wickets away to get a fine six for 51 from 17.2 overs. When the tour began, it was thought he'd have a battle to regain his test place; Graham Onions, with a strong showing in the New Zealand XI game in Queenstown, looked as if he would keep him out.
Instead Onions was sliced and diced by Rutherford and Co and Broad grabbed his chance.
Five years ago, he did well on the Basin in his second test. Maybe he just likes the place; he was hostile and persistent yesterday.
Broad was chuffed with his success, especially after arriving in New Zealand with worries over a nagging heel injury.
"I've felt decent rhythm throughout the whole tour. I hit the crease hard and today was just my day to get the nicks."
The second new ball wickets were important, enabling England to have the option of sending New Zealand in again. Players tend to play down the significance of the weather in their strategising. Not this time, Broad confirmed.
"It's not often enforced because bowlers tend to like a bit of rest and it's good to get their batsmen out in the field and build a big lead with scoreboard pressure," he said. "But with the weather around, it's unsure how much cricket is left in the next two days."
When Monty Panesar had Rutherford athletically caught at leg slip by Ian Bell early in the second innings, a slippery slope loomed.
However, Peter Fulton and Williamson knuckled down sensibly, adding 52. Fulton was strong off his legs, Williamson watchful.
There will be turn for Panesar today. In 12 overs, he conceded 13 runs, six of which came in one Fulton blow. He finished yesterday with four close catchers. A battle of attrition looms.
