Custom time balance requirement - YearTotal needs to be a separate value inputted by the user

Summary: Custom time balance: YearTotal needs to be a separate value.

Content (please ensure you mask any confidential information): We have a custom requirement to have yeartotal as an input field rather than Flow, balance, etc. We are trying "custom" time balance on account member as per the Oracle update. We are trying to bring Account1→Begbalance value on Account3→YearTotal field to achieve this. As per the screenshot, 1200 value should come on Account3→YearTotal. We have written a groovy rule for the same.

We are noticing that, if we write the rule to be Account3→May = Account1→Begbalance, it is working. It brings 1200 value on May period. But, if we run the rule for Account3→YearTotal = Account1→Begbalance, it doesn't work. In any case, yeartotal is always taking summation of Jan:Dec, working like flow.