PA28 PILOT FLYING EAST AT 6500 FEET HAS CRITICAL CONFLICT WITH A NW BOUND BARON ALSO AT 6500 FEET. BOTH ACFT ARE IN RADAR CONTACT AND TALKING TO CENTER.

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PRIOR TO FLT; I SPOKE TO A CO-WORKER WHO WAS GOING TO FLY ZZZ. I ASKED HIM WHAT ALT HE WOULD FLY AND HE SAID HE WOULD FLY AT 6500 FT. FOR SOME UNKNOWN REASON; HIS STATEMENT DID NOT TRIGGER A RESPONSE FROM ME THAT 6500 FT WOULD BE THE WRONG ALT FOR AN EBOUND FLT. SUBSEQUENT TO THIS CONVERSATION; I RETURNED TO THE FBO; WENT TO THE RESTROOM AND CALLED FSS FOR A BRIEFING. WX WAS VERY GOOD; WITH ONLY A POSSIBILITY OF CLOUDS 6000 FT SCATTERED IN E TX. I MENTALLY DECIDED TO FLY AT 5500 FT TO AVOID THE CLOUDS. PREFLT WAS ROUTINE; AND I TAXIED OUT FOR DEP. THERE WAS DIFFICULTY GETTING A PORTABLE GPS TO FUNCTION PROPERLY; AND I GAVE UP ON THAT AT THE RUN-UP AREA AFTER A FEW FRUSTRATING MINS. AFTER DEP; I COMMUNICATED WITH AN INBOUND FLT ON CTAF; AND THEN CALLED APCH FOR VFR FLT FOLLOWING. WHEN REQUESTING FLT FOLLOWING; FOR SOME UNKNOWN REASON; I RPTED CLBING TO 6500 FT INSTEAD OF 5500 FT. I RECEIVED A XPONDER CODE AND PROCEEDED E AS I CLBED TO 6500 FT. THERE WAS LITTLE TFC NEAR MY RTE OF FLT; AND I RECEIVED NO TA'S FROM APCH. I WAS HANDED OFF TO CTR FOR ADDITIONAL ADVISORIES. A FEW MINS LATER; I HEARD CTR ADVISING A BARON OF MY POS AND ALT; NOTING A DISTANCE OF ABOUT 10 MI. A FEW MINS LATER; CTR ADVISED ME THAT THE BARON WAS 4 MI AT 2 O'CLOCK POS; LEVEL AT 6500 FT. I RESPONDED 'LOOKING' AND ASKED TO REPEAT ALT. ABOUT 30 SECONDS LATER; AT ABOUT 1 MI DISTANCE; I SAW THE BARON. IT WAS HEADED NW. JUST AS I SAW THE BARON; CTR CALLED THE BARON WITH INSTRUCTIONS TO COMMENCE 'AN IMMEDIATE CLB;' WHICH HE BEGAN. I MADE AN IMMEDIATE R TURN OF ABOUT 45 DEGS TO EVADE THE BARON. I CALLED CTR AND SAID 'TFC IN SIGHT AND EVADED.' I ESTIMATE WE PASSED ABOUT 300 FT HORIZLY AND 300 FT VERTICALLY. THE BARON PLT SEEMED TO SEE MY AIRPLANE JUST AS WE PASSED EACH OTHER AT 180 DEG DIFFERENCE IN FLT PATH. AFTER A FEW QUIET SECONDS; HE COMMENTED THAT 6500 FT WAS THE WRONG ALT FOR AN EBOUND FLT. DUH! HE WAS RIGHT. MY AIRPLANE WAS AT THE WRONG ALT DUE TO MY ERROR; POSSIBLY THE RESULT OF THE SUGGESTION THAT MY CO-WORKER WOULD FLY EBOUND AT THAT ALT? OR MAYBE IT WAS THE FRUSTRATING TIME SPENT ON THE PORTABLE GPS? I RECALL THINKING THAT MY CO-WORKER MIGHT BE SELECTING 6500 FT TO STAY BELOW THE FLOOR OF THE LCL MOA'S (7000 FT) AS HE FLEW DIRECT TO ZZZ. I WAS SHOCKED THAT I HAD MADE THIS ERROR; AS I ALWAYS HAVE USED THE RULE '0-179' MEANS ODD ALT SINCE 179 IS AN ODD NUMBER AND 180 TO 359 IS EVEN ALT SINCE 180 IS AN EVEN NUMBER. THIS ERROR WAS MY RESPONSIBILITY; AND I WILL RECHK ALL PLANNED ALTS IN THE FUTURE TO ENSURE I DO NOT REPEAT THIS ERROR. A CONTRIBUTING FACTOR TO THE INCIDENT WAS THAT NEITHER CTLR NOTICED OR MENTIONED TO ME THAT I WAS AT THE WRONG ALT FOR DIRECTION OF FLT; AND IT DID NOT OCCUR TO ME EITHER.