sounds abotu right.
minimum thickness would be 20.4mm for 25kg, 450 diameter
if you're keen...
Radius^2 = 2.5^2 + (WEIGHT / (7.8*Pi*thickness) )
or change 2.5 or say 2.55 for a 51mm hole
I have both Ivanko and Hart plates at PTC.
The Hart 5kg ones are almost the same thickness, but cheaper. Buy those.
Beware, Hart allow 10% tolerance, so you WILL be getting heavier plates. I had to almost exchange every plate I bought to get them to weigh the same.
All my 20's are 21kg, some I got were 24.5kg lol