Wondering how much you have to pinpoint a vertical yagi on 2 meter?
No different than a horizontal antenna. Every antenna has published specs and one of those specs is half-power beamwidth. That is how wide the main lobe of the antenna is as measured between the two points where the signal is down by half which is 3 dB. Unless you plan on stacking some long boom yagis you need not worry too much. The Cushcraft 17B2 17 element yagi has a half power beamwidth of about 28 degrees by itself before stacking. Horizontal beamwidth only changes when antennas are stacked side by side. Stacking one above the other results in the same beamwidth as a single antenna but the elevation angle is lowered.