RPM measurement in 24V system

[M606BR]

Is there any trick to make possible RPM measurements using an automotive digital multimeter in a M38 with 24V electrical system?

I own an automotive digital multimeter which has TACH x10 function and all measurements I have made in my M38 indicated wrong RPM information. However, in my M606, which has 12V electrical system, the RPM information is always correct. My procedure for both cases was the following:

1) Connect the multimeter RED test lead to negative side of primary ignition coil;
2) Connect the multimeter BLACK test lead to vehicle ground;
3) Turn multimeter rotary switch to 4 CYL selection.

Notes:
For the M38 distributor I used the Distributor Primary Adapter No. 17-A-1375-50 which was screwed into distributor cover hole.
I have also made measurement by connecting the multimeter BLACK test lead directly to negative cable from the battery located in cowl, but the reading floats.
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Luiz Felipe Santos
1952 M38
1965 M606