Correct Input Sensitivity Setting for Cartridge

Greetings. I bought a 1978 vintage Sherwood receiver (as a second system) and am hooking it up to my Rega Planar 3 fitted with an Elys 2 MM cartridge. The cartridge has an input sensitivity of 4-6 mV, and the Sherwood phono hookup has a switch that moves to three positions, Low, Medium, and High. I set it to medium. Is that correct? Some people say this is not a big deal. The high sounds reasonable also but want to do it correctly. The Sherwood sounds amazing. Thanks in advance for your advice.
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Thanks for the info. As you can see, I haven't much experience with turntables. Formerly I just hooked the table to the Rega phono pre and that was it. Also I thought there may be some considerations about using a modern cart with an ancient receiver. 
Only in that the question arises whether the receiver is in tip-top operating condition.  Otherwise, some of those old classics can be great fun to use/
I settled on the medium setting as that sounded fine to me. Also, the Sherwood is completely restored and sounds amazing. I went vintage for the second system and for under $500 it sounds so good I am wondering why I ever spent so much on the fancy modern components. Some of the old speakers like the AR 5s, Polk Audio Monitor 7s, and Ohm Ls sound excellent, but that is another subject. Thanks all!