A C182 CFI CAUSES ANOTHER ACFT ON SHORT FINAL TO GO AROUND WHEN HE CROSSES THE HOLD SHORT LINE AT A NON TWR ARPT AT FNL; CO.

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I WAS READY TO TAKE OFF ON RWY 15 AT FNL. WINDS WERE FROM THE S AT 10 KTS. I CHKED FOR LNDG ACFT ON BASE AND FINAL. AFTER ASCERTAINING THAT THE AIRSPACE WAS CLR; I ANNOUNCED THAT I WAS TAKING OFF ON RWY 15 OVER CTAF. ABOUT 10 SECONDS LATER; ANOTHER PLT (ACFT #2) ANNOUNCED OVER CTAF THAT HE WAS GOING AROUND. I HAD NOT YET ENTERED THE RWY PROPER; BUT HAD CROSSED THE HOLD SHORT LINE. ANOTHER AIRPLANE BEHIND ME HAD ALREADY TAXIED UP TO THE HOLD SHORT LINE; SO I COULD NOT TURN AROUND. ACFT #2 HAD EVIDENTLY TURNED BASE NEARLY OVERHEAD; BASED UPON ITS POS WHEN I SAW IT. THE PLT HAD INTENDED TO LAND LONG; AND DID NOT EXTEND HIS DOWNWIND PAST THE APCH END OF RWY 15. (NOTE: THE LOGICAL EXIT FOR FULL STOP TFC ON RWY 15 IS 4500 FT PAST THE THRESHOLD; SO LONG LNDGS AT FNL ARE COMMON.) I DID NOT LOOK DIRECTLY OVERHEAD WHEN CLRING THE AIRSPACE BECAUSE I EXPECTED ALL LNDG TFC TO EXTEND PAST THE RWY END AT LEAST 1/4 MI AS PER THE AIM. PERSONALLY; I BELIEVE BASE LEGS SHOULD BE ABOUT 1/2 MI FROM THE THRESHOLD. EVEN 1/4 MI IS PRETTY CLOSE.