SAP has this simple short and sweet function module called
STRING_CENTER which does what it's name implies. Center a string, align in the middle. However: the function module is marked as obsolete and I can't find it's replacement/successor.. So here's the coding.
CLASS-METHODS: string_center changing str type any.
The method implemented:
METHOD string_center. DATA: lv_len type SY-FDPOS, lv_len2 type SY-FDPOS, lv_str type string. FIELD-SYMBOLS: <fs> type any. lv_str = str. lv_len = STRLEN( str ). move space to str. IF lv_len > 0. DESCRIBE FIELD str LENGTH lv_len2 in character mode. lv_len2 = ( lv_len2 - lv_len ) / 2. ASSIGN str+lv_len2(lv_len) TO <fs>. MOVE lv_str TO <fs>. ENDIF. ENDMETHOD.
Quite an ingenious bit of coding really... Thanks SAP !