How to set a value in a data field, when I need to perform a calculation based on the value of a field in another connected data object... using record rules

Hi need some advise...
I have 2 connected Data Objects. The first Data Object has a field called TOTALVALUE, the second Data Object has a field called CHARGE. When the TOTALVALUE in the 1st Data Object is above 10,000, then the value of CHARGE in the 2nd data object should be TOTALVALUE multiply by 0.05.
When I am trying to set the Record Rule to perform the above, I cannot see the TOTALVALUE data field (although I use the insert connected record option). How do I accomplish the above. Is there any article about how to handle such.... or any advise would be very much appreciated....

I have considered using Conditional rule in the data object, but the problem is when I need to amend the conditions related to a CHARGE, the values in the future, all my previous CHARGE value would also change - this would be a problem in my case, as I want the previous value to remain, and only have the change impact the new records.

TQ.