how to enter values for exact positioning?

 From:  mike (MIOHN)

just playing around with MOI a bit again and I'm wondering
if its possible to place/position objects via entering exact values?
F.e. I make a circle with 10cm and a second circle with 5cm and later, I
want to place the second circle 8,34 cm above the first one.
Is there a simpler workaround for this - other than making a verticle line
of that lentgh and snapping the circle at the end of that line?

Or, how can I "later" change the size of a circle-spline by entering/changing
the radius?

Sorry, seems a stupid question but I cannot find any threads about that.