Hi John,
If you add a multi-choice field called something like “Balanced” with two options “Yes/No” and then set two conditional rules on it.
The rule will be based on a field value.
https://learn.knack.com/article/oaj8ajhwkc-conditional-rules#adding_conditional_rules
The first states that if the total equals the calculated sub totals then the value is”Yes”.
The other is the opposite, so equals “No” if the total does not equal the calculated sub totals.
You can then either show the “Yes/No” field in the table or just use a display rule on one of the values to turn the field red or green (or show an icon etc) based on whether the multi choice field is “Yes”’or “No”. This will give the user a visual clue that there is an issue.
You could then run a task, if you are on a Pro account or above, to look for any values that are “Yes” and send an email to notify on a daily schedule.
If you are trying to validate that the two sub totals equal the manually added total before the form is submitted this will require the skills of a JavaScript person to create a query before the record is added……that’s not me 
Hope this helps.