If an item is called a "straightedge" it is implied that it is straight, but when you find out that the thing isn't straight... well, then that completely defeats its purpose and renders it useless...
I have a Veritas straightedge and it alings perfectly with my guitar straightedge from Stewmac. Both cost approx. 100Eur a piece.
The Harley Benton Bass straightedge, costing 1/3 of the other two, is measurably not straight. When I hold it against my Veritas straightedge the notches on the "Harley Benton Bass Edge" have various degrees of light shining through underneath.
I love the idea that you can get a usable tool for less €, but in this case I guess you really have to spend more, in order to get a proper straightedge.
I cannot recommend this tool as it does not fulfill its purpose.