As the new Report Transformations - No-code development platform does not support rounding, I will not pursue this approach. I have no use for a report transformation which generates false annual tax return accounting figures.
Two custom fields on accounts, a custom report, external rounding followed by simple arithmetic results in correct accounting figures, so would be used instead.
With Report transformation specifiable rounding and simple arithmetic, One custom field on accounts, a custom report to check allocation, and a report transformation could be used instead.