We can't speak to the specific endmill you are using, but can provide some general information about tool deflection. In general, anything that increases rigidity of the cutter will minimize deflection and vice versa. Mathematically, deflection is directly proportional to the cube of the length divided by the fourth power of the diameter.
Practically speaking, with all else equal, cutter diameter will have the largest impact on deflection (bigger being better), followed by stickout (the amount of cutter extending past the collet) which should be minimized, and finally number of flutes (fewer cuts in the metal = more rigidity).
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Anyone having a deflection problem with onsrud compression 1/4 bit