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Somewhere in that 20 - 30 degree range would be CORRECT RUNNING TIMING. (I don't know offhand what the exact lag degree spec of your impulse would be.)
At cranking speed, the IMPULSE COUPLING in the mag catches, winds up a spring for 20 or so degrees of crankshaft rotation, and then "trips", giving you a HOT spark at about 0 degrees.
This does 2 things... the quick snap of the spring gives a hot spark from the mag to aid in starting, and the winding up of the spring retards the spark timing for starting. This "static" setting at about 0 degrees is important because if the engine were ever to be hand cranked, you do not want a spark before 0 degrees (TDC), as a "kickback" will occur, likely to injure the person cranking! When these tractors were made, the manual usually referred to "static" timing", as timing lights were not commonly available in farm settings, and the timing could easily be set by slowly rotating the engine with the spark disabled, and the point at which the impulse coupling "snapped" could be referenced against a timing mark. As the engine "catches", and speeds up, centrifugal force acts on weights in the impulse coupling, and swings them to a position where they do not "catch", disabling the snap action, and the timing advances all at once to the running timing. In that way, the timing on these "mag" engines differs from engines with a distributor. With a distributor, the spark advance occurs from idle up towards governed speed at a rate determined by the "ADVANCE CURVE", which is set up by the manufacturer's choice of weights and springs in the advanve mechanism. To sum this up...
If you set the static timing to the manufacturer's spec for static timing (X number of degrees, or a mark labeled "spark") at which the impulse should trip as the engine is SLOWLY turned over, the running timing WILL BE CORRECT, ASSUMING THE CORRECT IMPULSE COUPLING IS PRESENT ON THE MAG, AND IS FUNCTIONING PROPERLY.
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