INSTRUCTOR WITH SPI MISREADS DME; ENTERS TCA WITHOUT CLRNC.

Narrative

I WAS RETURNING FROM FREDERICK; MD; ON A TRAINING FLT WITH A STUDENT; TO MARTIN STATE ARPT IN BALTIMORE. AFTER DOING SOME SOME AIRWORK I CALLED BALTIMORE APCH TO REQUEST FLT FOLLOWING AND TCA CLRNC TO DSND INTO MARTIN. I RPTED MY POS AS BEING 29 MI NW OF BWI; AT 3500 FT. AFTER ASSIGNING ME A TRANSPONDER CODE THE CTLR INFORMED ME THAT I WAS 18.7 MI FROM BWI AND IN THE TCA. HE FURTHER STATED THAT SINCE HE KNEW MY ACFT WAS BASED AT MTN AND SPOKE WITH ME FREQUENTLY; NO RPT WOULD BE MADE AND THE MATTER WOULD GO NO FURTHER. HE GAVE ME THE TCA CLRNC AND I THANKED HIM FOR HIS CONSIDERATION. I DID NOT INFORM HIM THAT ALTHOUGH I DID MISREAD MY DME; I WAS TECHNICALLY NOT IN THE TCA SINCE I WAS BELOW THE FLOOR OF 3500 FT. ALTHOUGH I RPTED '3500 FT' ON MY INITIAL CALL IN; MY ALTIMETER AT THE TIME OF MY DEP FROM BOTH MARTIN AND FDK INDICATED 50 FT HIGHER THAN FIELD ELEVATION WHEN SET TO CURRENT BAROMETRIC PRESSURE. I AGAIN VERIFIED THIS UPON LNDG AT MTN; WITH REF TO THE CURRENT ATIS. THUS; I WAS EXACTLY AT 3450 FT; AND ALTHOUGH I INADVERTENTLY FLEW WITHIN THE 20 MI RING; I WAS BELOW THE 3500 FT FLOOR. HUMAN FACTORS: IT WAS BASICALLY GOOD LUCK THAT ALLOWED ME TO REMAIN BELOW THE TCA (TECHNICALLY). FOCUSING TOO MUCH ATTN ON MY STUDENT AND IGNORING NAV ALLOWED ME TO TEMPORARILY LOSE POSITIONAL AWARENESS; WHICH IS A SERIOUS ERROR ON MY PART. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. CALLBACK MADE TO COUNSEL RPTR REF THE FACT THAT ATC USES 3500 FT. RPTR STATES THIS IS NOT NORMAL PROC FOR HIM. USUALLY ALLOWS AT LEAST 500 FT BELOW TCA FLOOR. KNOWS HE WAS IN ERROR.