I would suspect a late '80s Strada would be 26.8 and I agree starting with what is printed on the current post is always your best bet, unless it fits extremely loose or tight.
Scooper those are great tools. I don't use mine often but much better than the caliper approach.
__________________
“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk