This is what works for me currently, there are other perfectly good ways that work for others, blah, blah, blah,...
If it's warmer than about 40 F, if I wear any shirt I'll sweat, so I wear nothing. If it's raining it won't matter that I get wet. I just ignore any comments about the old guy not wearing a shirt : )
If I get cold and/or it starts raining I'll put on my hooded WPB jacket. Sometimes I'll put hood over head and don't put arms into sleaves. If it's not raining too hard/windy I'll unzip the front.
Sometimes it'll stop raining and I get a little warm, so I'll take off WPB jacket, then I'll get cold or it will start raining a little more and I'll have to put WPB jacket back on. Maybe I walk a little faster or slower. Maybe it would be a little more convenient to have a windshirt in this situation.
If I'm hiking and above 20 F, just shirt and WPB jacket is sufficient. May have to walk briskly to stay warm. I never go below 20 F.
When I stop hiking and I need warmth, I put on synthetic or down vest. The warmth per ounce is better for synthetic or down than having extra layers of windshirt, mid layer, or fleece. I have a lighter vest for warmer weather and a heavier vest for colder weather.