How should I set InitiatingHeapOccupancyPercent of G1GC for NameNode with high usage of jvm heap

[#1]

Now I are trying to change GC from CMS to G1GC.

And let's say, current situation of NameNode with CMS are

Physical memory size : 140 GB
-Xmx100G -Xms100G
current actual heap usage : 70 - 80 GB (So, usage is around 80%.)
-XX:InitiatingHeapOccupancyPercent : 90

The default value of "-XX:InitiatingHeapOccupancyPercent" is 45. If I set 45% for "-XX:InitiatingHeapOccupancyPercent" for this NameNode, I think, current heap usage always hits the threshold...

Could you advice how I should tune "-XX:InitiatingHeapOccupancyPercent" for this NameNode?

[#2]

Sorry, There was copy and paste miss in question above. Now we use "-XX:CMSInitiatingOccupancyFraction=90" for current NameNode with CMS.

[#3]

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