Yeh…not sure why I didn’t think of the simple solution…over thinking I guess. Well that’s probably almost 2 years I’ve thinking of this. Facom nice, looks like it‘ll workDoesn't necessarily need a ruler. Many variants of the tool just use a pointer.
I don't recall there being anything particular about the rule. Just measure the width of the locating slot and procure a 6" steel rule that will fit.
The Facom one at around £1.50 is a beaut and cheap (and available all over the show)
https://uk.rs-online.com/web/c/hand...150mm+steel+rule&searchType=CATCH_ALL_DEFAULT