Via fence - Thumb Rules

Thumb rules:

Stitching Vias for RF should be spaced at lamba/20, where lamba equals the wavelength of the highest frequency of interest.

For striplines, "a rule of thumb is to place the fences at least four times the trace to ground plane distance away from the line being guarded"

Source: Via fence - Wikipedia:

Differential Pair Routing

"This application note discusses the selection and optimal settings of differential pair design rules. When it comes to successfully deploying differential signals in high-speed designs, symmetry is the key."

Barry sums it up nicely.

"In Conclusion

1. Symmetry is the key to successfully deploying differential signals in high-speed designs. Maintaining the equal and opposite amplitude and timing relationship is the principal concept when using differential pairs. 

2. Match the length of each signal of the pair. This ensures that there is no skew between the signals of the pair and flight times will be identical.

3. Route the differential pairs to impedance and at the optimal spacing: Gap = 2 x trace width.

4. To control crosstalk, keep aggressors far away from differential pairs, especially on 
Microstrip (outer layers). A good rule of thumb here is Gap = 3 x trace width. PCB"

Source Link: Differential Pair Routing - Barry Olney 

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