The ‘notch’ is really a little file cut - and I don’t think it was always used, but I have seen it on some of my clocks.  It was not very accurate because things varied.  A variety of bob shapes and variations have been used; normal and shell type (at least two versions of shell type), iron/steel, lead, brass, possible other combinations (I have heard suggested brass case, lead fill).  In addition - various other ‘additions’ are occasionally used on the rod; weight trays are common, seconds switch ’trip’ is used on seconds clocks, synchroniser spring assembly is used on a few clocks etc, so it is only a very approximate mark for initial set up.  If you start by putting the rating nut about half way up the available thread, you will soon find you can work from there.  As you get close - you have to take more time, but the initial ‘coarse’ adjustment can be done simply by comparison with a seconds equipped radio controlled clock and typical can be got within a minute a day or so quite easily.  More patience is needed for the fine adjustment!  Very fine ’trimming’ can be achieved by a 4 or 6 BA washer (or two) on the bob top.