I am often dismayed to learn the logical results of my ideas. :)

"If I spend 20% of my life on growing petunias, am I thus saying that anyone who doesn't grow petunias is less valuable?"

I think so. You might believe that other people have different value systems that are equally valid. I think that the question of how to compare or combine the values of different people is a different question.

In most situations, if you weigh your life vs. other peoples' lives, it would seem you should assign your life a much higher value. The stranger your values are, the higher you should value your life wrt other people. That's because your life is directed towards your values, and other peoples' lives are not. So altruism, of the type explained by kin selection, is immoral. But this is countered by the fact that, the stranger your values are, the more you should discount your own values wrt the values of others. You'd probably have to formalize it to figure out which factor predominates.

"does this imply that a life of a newborn baby -- my baby! -- is worth exactly zero, because she hasn't experienced anything except her mother's womb?"

The way I said it does. I was being sloppy. But even if I revise it so that the infant has some non-zero value, it wouldn't be satisfactory; because parents have it biologically programmed into them to assign extra value to their own children. We would have to address the problem of combining different peoples' values. And I'm not going to address that problem now.