The 1/10 current proportion is called "hard saturation" as opposed to "soft saturation" wich would be to drive the transistor with just the necessary current to drive the load.
i.e: if a transistor has a gain of 100 you would only need a 100 ohms input resistor to drive a 1 ohm load, but in practice not every transistor would have an exact 100 gain, some will have more, some less, so with the 100 resistor some transistors would be under driven. So when we need them to act as switches, fully on or fully off, the best approach is to "hard saturate" them to make sure they operate as we expect.
i.e: if a transistor has a gain of 100 you would only need a 100 ohms input resistor to drive a 1 ohm load, but in practice not every transistor would have an exact 100 gain, some will have more, some less, so with the 100 resistor some transistors would be under driven. So when we need them to act as switches, fully on or fully off, the best approach is to "hard saturate" them to make sure they operate as we expect.
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