Notched straight edges are horribly expensive. I'm thinking of getting an aluminium ruler and filing out the notches as a cheaper alternative. I realise this wouldn't be precision ground to a thousandth of a cats whisker, but surely it would be good enough to do the job. Wouldn't it?!   Something like this - https://www.amazon.co.uk/Faithfull-RULE1000-Aluminium-Rule-39-inch/dp/B000C750F6/