You still think in 1-based system, Pi unit * Pi unit is Pi of Pi units or 3.14159.. Pi units.
Also, Pi unit / Pi unit is 1/Pi Pi units or 0.318309886183790.. Pi units..
1 is also a number, a number we chose by convention to be a base unit for all numbers. You can break down every number down to this unit.
20 is 20 1s. 1.5 is 1 and a half 1.
If we have Pi as a unit, circumference of a circle would be radius*2 of Pi units. But everything that doesn't involve Pi would be a fraction of Pi, e.g. a normal 1 is roughly 1/3 of Pi units, 314 is roughly 100 Pi units, etc. etc.