Running Total including Null values

[Spolzie]

I need help with a simple formula:

I have attached a spreadsheet example.

Concept:

Total Charge

Three Savings Buckets A, B, C

Bucket A Savings = Total Charge minus Bucket A Allowance

Bucket B may be null with no activity so

when I start with Bucket C, formula needs to start with the Bucket A Allowance

All Buckets with values
Charge A Allow A Save B Allow B Save C Allow C Save
500 350 150 300 50 100 200
500 - 350 350 - 300 300 - 100

Bucket with Null Value

Charge A Allow A Save B Allow B Save C Allow C Save
500 350 150 Null Null 300 50
500- 350 350 - 300

Need formula sees null and takes last subtotal > 0 and Null = no entry at all not zero