I can’t find any guide tool or menu item that allows me to add guides
have a look at the ‘Tape measure Tool’…
Or get a construction line plugin - search Extension Warehouse or Sketchucation Plugin Store for construction line or construction points or guidelines.There are several but I can’t remember them by name, and am not at my computer at the moment (on my phone).
Post again here if you have trouble finding one to suit, and I’ll try to help further.
This one might be suitable, but there are others: