Why in the world are you assuming that any estimate is possible? Most people have a fixed number they can be expected to make based on their current wages. You don’t guess that you’re going to be promoted or get a raise any more than you guess that you’re going to lose your job. You take your weekly earnings and multiply.
Sure, if you run your own business or work on commission, there might be allowances, but most people work for either a salary or a wage.
There was a discussion I heard about AHC Act, actually quite reasoned, I believe it was because it was on Canadian radio. One commentator mentioned that typically, with a huge complex bill like AHC - and he mentioned a half dozen different bills on various subjects since the 1960’s - as the implementation happens, there are a number of loopholes, contradictions, and “this doesn’t work” situations that come up. In previous cases, the committees in congress get together and iron out compromises that fix the problem and pass amending legislation. Certainly, having a key requirement like Medicare expansion as part of the plan, then subsequently removed by Supreme Court decision, qualifies as a shortcoming in the implementation. The obvious solution is to extend the subsidies to lower incomes where Medicare does not cover. However, the Republican majority in the house will consider no other changes but repeal - so these holes live on.
For example, another flaw was the cost of family support. The rules set a limit for individual support cost as a percentage of income, but none for family - since family costs could be quite variable, some families the spouse may have coverage elsewhere, etc. The simplest way for employers to game the system is to low-ball employee personal coverage, and set family premiums much higher. The employee is then screwed - if they go to the exchange, they do not qualify for subsidies because of the low cost of employer personal coverage, even though the rest is unaffordable.
I’m sure there are plenty of other holes that congress could fix if they wanted to. I haven’t seen a groundswell of opinion that the pre-existing condition rules should be repealed…
Some people work seasonally, or the amount of work they do is based on the amount of hours they are assigned (which varies), or they bounce from one job to the next with periods of unemployment in the middle.
… or there are bonuses, or overtime, or second job here and there…
I can honestly say that every year I paid taxes (30+ so far) I could not accurately estimate at start of the year, within $1000 or so, or even $5K, what my income for the year would be.