Enquiry of vacuum box size setting in HFFS v11

Hi everybody,

I'm not sure whether my calculation of vacuum box size is correct or not ?

my calculation:

my design is tag antenna in UHF (908.5 - 914 MHz)

v = f x λ
=>3e8 = 910e6 x λ
=>λ = 330mm

if my antenna size is 40(X) x 50(Y) x 1.6(Z) mm
then, the vacuum box size is:

[(λ/4) + edge of XZ plane] x 2
x
[(λ/4) + edge of YZ plane] x 2
x
[(λ/4) + edge of XY plane] x2

Is it right?

Also, I don't know why I get two impedance response (pls check attachment). Is it correct?