(Photo by Jon Elswick/AP)

The Obama administration on Monday announced that 5 million people had signed up for Obamacare exchange plans. Hours earlier, a self-employed Web developer from Michigan had already predicted the milestone would be hit on Monday.

Meet Charles Gaba: He’s not a professional statistician, heath-care expert or political operative. He’s a self-described “numbers geek” who just wants to know how the new health-care law is doing.

He’s been tracking the most up-to-date enrollment information and offering his own projections on his blog, ACAsignups.net. On the same day that he predicted the 5 million signups milestone, he accurately predicted that California would hit the 1 million mark. For policy wonks and health-care journalists who have clamored for more information about Obamacare enrollment, Gaba’s blog has become a must-read.

His next big prediction: The final sign-up tally will hit 6.22 million.

The big political focus has been on whether exchange enrollment by March 31 will reach 6 million, the revised estimate provided by the Congressional Budget Office. After the Department of Health and Human Services announced 4.2 million sign-ups through the end of February, the Avalere Health consulting firm projected that sign-ups would hit 5.4 million by the deadline.

Our typical disclaimer applies here: The CBO projection is for official enrollments. The HHS numbers, as well as Avalere’s and Gaba’s, are all counting sign-ups, and not whether someone has completed enrollment by paying the first month’s premium. By the best estimates, anywhere from 80 to 90 percent of people signing up have paid their premiums.

Gaba admits there’s some guesswork involved in his projections. He’s pulling together information from monthly HHS enrollment reports and the more frequent updates from the states running their own exchanges.

For instance, reports from California’s exchange indicate that average daily enrollment there has more than doubled in the first half of March, compared with February. Colorado and Kentucky also are on pace to enroll about twice as many people this month:

(Source: ACAsignups.net)

The one number that Gaba has struggled to nail down is off-exchange enrollment – people who signed up directly through an insurer. That option may have been particularly attractive to people who didn’t qualify for federal subsidies and wanted to avoid HealthCare.gov.

“That is the single, biggest untold story of the numbers right now,” he said.

Gaba doesn’t just track Obamacare enrollment – he’s also a customer. He signed up through HealthCare.gov after his plan was cancelled, but the experience wasn’t easy for him.

He failed to make it through the system on Oct. 1, the day the Web site launched and quickly crashed. He tried again in early December and was able to sign up without issue, but he said it took almost two months for his insurer to confirm his enrollment. He said his new plan costs more, but he figures that its better coverage evens things out.

He admits that he does have a rooting interest in seeing the law succeed – he’s a volunteer for the local Democratic Party. Still, he says he is just trying to figure out whether the law is working.

“I do think the good outweighs the bad, but I don’t think [Obamacare] is the greatest thing in the world," he said. "I’m a single-payer guy."

At first, Gaba spent about an hour each day on his enrollment blog. In the past few weeks, that's jumped to several hours a day.

“My business is starting to suffer here,” he said.