BULGARIA - PENAL PROCEDURE CODE
Part two. EVIDENCING
Chapter thirty six. PROCEDURES IN CONNECTION WITH INTERNATIONAL COOPERATION ON PENAL CASES
Section 1 Transfer of Convicted Persons
Art. 457. (5) The pre-trial detention and the served punishment in the state where the sentence has been enacted shall be deducted, and when the punishments are different, these periods shall be taken into consideration in determining the term of the punishment.
Reduction of national sentence following pre-trial detention - national proceedings
EDIT.