One needs to keep in mind the via to via spacing allowed in the design.
If your 35 mil vias can reside on 50 mil centers, a row of vias will
effectively build a moat in the plane if you use an anti pad of 55 mils.
This is why we use a 45 mil anti pad on our vias.

Another 2 cents worth...

Ken Barrett
Cisco Systems
San Jose CA
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408.526.5625

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