[Moon] dish question

[SM5LE]

Hi all
 
If I use a small dish like 2.2 m; how much do I gain to go to offset dish (with zero blocking) ?
I am using a 42 cm ring and did this calculation. 
Can I compare square cm:s when calculating the blocking from ring ?
 
My choke-ring blockage. (In theory) :

2.2 m dish is 38013 cm2 (my dish).
Area for ring (42cm diameter) is 1385 cm2.
Fictive area (dish with ring): 38013-1385 =36628 cm2
Fictive dish diameter (dish with ring); radius is: r2 = 36628/2π = 36628/6.28 =5833 and √5833=76cm. Gives diameter = 1.52 m
Gain for 2.2m 26.73dBi (VK3UM software) 
Gain for 1.5 m 23.31dBi (VK3UM software) 
and finally; ring blockage: 26.73-23.31= 3.42dB
 
Is it realy that much , 3.4 dB ... 
 
Tnx for some answer
/Sven SM5LE


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