Credo HX and NF rings

[Bengradillas]

Looking to get some NF rings for my credo HX 42mm on a Tikka T3X build. Which height would you recommend?

 

[Macintosh]

What stock?
NF UL 30mm lows on the oem CTR rail fit my x42 scope perfectly, but its not a particularly high-combed stock.

 

[Bengradillas]

Tikka T3X stock for now. Would like to upgrade in the future. You were able to mount the NF directly without a rail?