(1) Not more than shaft diameter x 1
(2) Not less than amount of forward axial movement + 2mm

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I would suggest in some circumstances these do not work out as the best solution.

There needs to be adequate gap between the propeller and cutless bearing for a free flow of water to flood and circulate in the bearing and I feel 2mm does not meet this requirement. In fact one only has to do an eyball survey around a marina to see that most folk agree with me and it seems that a normal gap is around 2/3D on a 1" shaft .

If the propeller sits inside a cut out then this also may be a determining factor