This is another of those "multiple aggregation" problems that come up on the forum from time to time, isn't it?
Only way I can think of is by doing the cumulative sums in custom SQL rather than using table calculations.
Upfront, Hope I did not miss anything concerning your challenge but find my approach as reference below and stored in attached workbook version 2018.2 located in the original thread Cumulative Running Sum Percent Calculation
1. M1. A summ: RUNNING_SUM(SUM([A]))
2. M2. B summ: RUNNING_SUM(SUM([B]))
3. M3. %: [M2. B summ]/[M1. A summ]
Hope it helps,