ALTDEV ALT OVERSHOT BY PLT OF AN SMA ACFT DURING DSCNT IN IMC.

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ON FLT FROM EUGENE; OR; TO CRESCENT CITY; CA; VIA NORTH BEND; OR; FRONTAL ACTIVITY WITH FREEZING LEVEL PREDICTED TO 6000-7000 FT. FILED IFR AT 5000 FT. AFTER DEP FROM EUGENE; THE ACFT AHEAD OF ME WENT TO 7000 FT TO AVOID CUMULO NIMBUS AND I ALSO THOUGHT I COULD DEVIATE AROUND THE CLOUDS BETTER AT THAT ALT. WAS CLRED TO 7000 FT; THEN 9000 FT BY ATC; BY REACHING NORTH BEND; OR. MODERATE TURB AND RAIN ENCOUNTERED. TURNING S AT NORTH BEND; APPARENT THAT I COULDN'T TOP CUMULO NIMBUS; SO WENT TO 7000 FT MEA; ASSIGNED BY ATC. BTWN NORTH BEND AND CRESCENT CITY ENCOUNTERED MORE TURB; RAIN; AND RIME ICE ON SPINNERS AND WINDSHIELD. RPTED CONDITIONS TO ATC AND; AS NEARING CRESCENT CITY; WAS ASSIGNED LOWER AT 6000 FT; THEN 4000 FT. SUDDENLY VISUAL AT 4000 FT ABOUT 20 DME NORTH OF CRESCENT CITY DEST. I HAD NEVER BEEN THERE BEFORE. THE ARPT WAS UNDER A CLOUD DECK AHEAD; ABOUT 500 FT CEILING. MY COMMERCIAL IAP MANUAL HAD BEEN KNOCKED TO THE FLOOR (TURB) AND I HADN'T HAD TIME OR ABILITY TO STUDY AN APCH PLATE; WHEN ATC ASKED ME WHICH APCH I WANTED! FUMBLED THROUGH THE APCH PLATES; SELECTED THE VOR FOR RWY 11; ADVISED ATC; AND LOST 200 FT OR MORE (MAYBE 300 FT) FROM 4000 FT ASSIGNED ALT WHILE RETRIEVING THE MANUAL AND FINDING THE SEVERAL APCH PLATES. I WAS HAND FLYING BECAUSE OF THE TURB; AND TOO BUSY! DUMB MISTAKE.