The temps you mention sound like the temps I had on my Spring tour when starting in the early darkness. I started in shorts and a short sleeved jersey with a wind breaker over it. As the day warmed up I took off the windbreaker. That was fine for me. My legs might have been slightly chilly when starting but quickly warmed up. The coldest mornings were 40 (F) or so.
If I wanted a bit more warmth I'd probably use either thin tights or leg warmers (maybe 40 to the mid 30s).
If it was substantially colder (lower 30s?) I have some tights that are slightly brushed inside. They are substantially warmer.
If it were really cold I have some tights that are windproof in the front they are good in bitter cold weather (0 and single digits), but I'd probably never take them on tour.
Generally I would guess what the temps would be and the the one of the three that seemed to make sense. Worst case I'd wear the rain pants or have something mailed from home. That hasn't happened yet though.
I carry light weight rain pants but have never worn them while riding. It is nice to be able to be dry in camp in the evening though.
On the thin tights vs leg warmers issue...
Leg warmers are easy to take off as it warms up and lighter.
On the other hand tights were nice to wear under my zip off leg pants in camp on cold evenings.
Wearing bike shorts all the time is not acceptable to me I need a break from them in the evening.