However her efforts are allocated, they are "her interests" because she chose them. I'd say that the reason she chose them to be her interests is the key. Why did Sister Teresa chose her life? Because she believed that the well-being of those she served mattered more than her well-being. She had accepted a moral belief that compelled her to make sacrifices. I have accepted a moral belief that the well-being of others does not arise higher than my well-being. I think that this is the way to answer the question of whether an act, or a chosen "interest" is altruistic or not. What is the moral belief behind the choice of the interest, or the act?
When you talked about the percentage of interests in your example:
Consider an example of a woman, who has a husband, children, and works part-time. Assume she estimates that in recent months 51% of her waking time or efforts are for her own interests, and 49% for the interests of others -- her husband, children, and employer. The 51% makes her interests primary. She seems to qualify as an egoist on your terms. I'd say this woman was an altruist because she appears to have accepted, and acted upon, the belief that she has a moral obligation to sacrifice some part of her life for others. If this is the case, the particular percentages are more an accident of her schedule, than a determiner of whether or not she is an altruist. Now, if she loves her husband and kids, and enjoys taking care of them, then she is behaving egoistically - and those numbers would be meaningless.
If she believes that her husbands well-being is more important than her own, but she enjoys cooking his dinner, then it just gets harder to categorize that act. It would have to be a very sad altruist that finds no enjoyment in any of their acts, and a very unusual egoist that whose acts have never benefited anyone else.
I think it has to be the conscious or subconscious belief that drives a person's particular action that determines if they are behaving altruistically. This moves the issue towards measuring the belief system as opposed to the individual action. The action just can't be measured in the absence of the belief that prompted it - especially since most people have mixed motives - one time acting on rational self-interest (which might help another person, who they value, or not) and another time acting out of a sense of duty to another's well-being over their own. And the motivation, or driving force behind the act matters, since some acts are more difficult than others and must be compelled by the moral belief.
As to labeling a person an altruist... that gets complicated.
But there are other issues like how intensely they hold their beliefs, do they compartmentalize their altruistic beliefs so they only apply in certain areas of their life, and, being into psychology, I'm always curious about an altruist's self-esteem. Do they experience themselves as less worthy than others? Is a strong sense of duty held in a quasi-heroic fashion, or is it held more like a burden they are punishing themselves with?
- It could mean "Someone who advocates altruism" - whether they actually practice it or not.
- It could mean someone who believes that they have a duty to act for the well-being of other over self - again, whether they actually practice their beliefs or not (some people tell themselves lies so they can pretend they are following their beliefs when they really aren't).
- It could mean someone whose acts are motivated by altruistic beliefs greater than 50% of the time.
I think that the bottom line on judging an act altruistic is going to require that there be a sense of sacrifice. There must be acts that bring their well-being into conflict with another's. Without that a person could be enjoying helping another in some way that isn't costing them as much as the enjoyment they are experiencing. [Another measure in all of this might be where a person chooses to make their whole life about helping others - like a nun, or priest. There the sacrifice is not just in this or that act... but also in the life they could have experienced and the person they could have been. In other words, altruism can be more than a standard for choosing in a given instance, but it can also be a goal... something where the person actively seeks instances to be altruistic, and where the person seeks to become more altruistic in their nature. That's a different level, but it is still about their belief and their motivation.]