Matching Corner Radius to Multiple Size Objects

[PR_.DDY]

I can't figure out how to match these corners accurately. 

Okay so I have a 12"w x 18"h rectangle (white) with .5" radius corners. 

Inside of that I have a 11"w x 17" rectangle (blue stroke) with .5" radius corners. 

HOWEVER, they do not match and look very funky. 

Do you have any advice?

[Ton Frederiks]

I gave your outer shape a stroke (on the inside) of 30 pt to show what the rounding would become on the inside, a very tiny one for the red one 0,005 in (without the  round corners effect).

 

[Jacob Bugge]

PR,

As I (mis)understand it, you will be happier if you apply a difference in (outer) corner radius that is half the difference in rectangle size.

So with the larger being 12" x 18" and the smaller being 11" x 17", the difference is 1", so the difference in radius should be 0.5"; you can use 1" for the larger and 0.5" for the smaller (or 0.75" and 0.25" or similar; if you go down to 0.5" and 0" with a centre aligned stroke for the smaller you need to use Round Join).