I don't know a darn thing about what I am about to say but since you have no takers this can't hurt.
I assume the buzz box is connected to the (wire) ignition point input to the distributor. This input wire has voltage on it until the points close and when they open again it has voltage again. Voltage powers the buzzer and you hear a buzz with voltage present.
Ignition systems generate a spark when the points open after being closed (called dwell time which allows the ignition coil to fill with energy which fires the plugs).
Most engines run at TDC or B(before)TDC as you have to bust the gas before the piston gets to the top, otherwise you loose part of the power in the stroke.
The magneto is probably connected to your throttle linkage, as well as belt connected to the crankshaft, and varies in timing as you advance the throttle, like 4 degrees BTDC at idle to maybe 20 BTDC or so at WOT.
Most Mercs turn CW (viewed looking down on the crankshaft) when they run. If I wanted spark before TDC then I would want the points to open .235 inches before TDC with the crankshaft turning CW.
In summary set the magneto so that as you turn the crankshaft CW, as you move toward TDC, the points being previously closed and open (you get a buzz aka voltage to produce the buzz) .235 inches before you reach TDC. Where do you measure .235? I dunno! But I'd bet if you had marks on your flywheel, it's be about 4 degrees BTDC.
Hope that helps. Best I can do.
PS. Just had a brain fart! The .235 is measuring the crown of #1 piston coming up on the compression stroke just before TDC which would be considered 0 for the purpose of measuring the position of the piston. Where else would you have a decimal measurement? That makes sense! Since the stroke is known and happens once per rev of 360 degrees, .235 can be easily converted to degrees BTDC.
If you know the stroke or measure it, piston at the bottom, and piston at the top, do the math.
Say the stroke is 2.13 (stroke of the old classic Merc 4 cyl cross flow which the 45 is probably a decendant). Since you are going from the bottom to the top, you are only moving 180 degrees of crank rotation, not the full 360. So .235/2.13 x 180 = 20 degrees. That would be your WOT setting or real close. (4 and 20 were the numbers for my last Merc.) Your idle setting would be for a guess at 4 degrees so that would be x/2.13 = 4/180 = .047".
So, if this makes any sense, your .235 is WOT and .047 would be at idle.....piston crown position when the buzzer buzzes, as you rotate the crankshaft CW, vs the piston crown position when at TDC.
Whadda-ya-think?
