0x18 hex is 24 decimal, its just the same value represented two different ways.
The 24 is the position (offset) of the data byte in the data steam packet. That value is called the "Start" value in EFILive, so put 24 in "Start".
The source data size is the same as EFILive's "Bits" field.

Note: If you have a 16 bit field, you have to specify the second byte as the start byte in EFILive, I'm not sure if the other package specifies the first byte.
For example, say the RPM was 16 bits and the two 8 bit bytes took byte positions 14 and 15 in the data stream.
In EFILive you would need to put 15 as the start byte and 16 as the bit size.
In the other package, they may specify it as an offset of 14 - just something to watch out for.

Regards
Paul