I've got that T-square.. and basically any standard 0.5 pencil works like a charm.. (i've also got the smaller 85mm version which I use quite often) again..standard 0.5mm mechanical pencil is good.. Personally I'm using pencils with metal tip rather than plastic
precision wise...
0.5mm+/- in woodworking... that would be really nit picking..
for better precision we have CNC machines
the good thing about the t-square is it is nice 90deg and pretty precise across 600mm ...definitely speeds up some measurement and marking processes..