TL;DR: In the year following your exit from an actual tax regime, you reduce the relevant revenues by the amount excluding tax of the receivables shown on the balance sheet for the last financial year taxed under this regime, then apply the 87% allowance.
The year concerned
The rule applies to the year following your exit from the actual tax regime.
Receivables to deduct
You deduct from the relevant revenues the amount excluding tax of the receivables shown on the balance sheet for the last financial year taxed under the actual tax regime. This deduction therefore does not cover all receivables from your business indiscriminately.
The order of calculation
You must first reduce the relevant revenues by the amount excluding tax of the receivables taken into account. You then apply the 87% allowance to the amount thus obtained.