Best way to scribe straight line down centre of tapered bar?

[DarrenUK]

Can anyone suggest a tool or a quick easy way that I can use to scribe a straight line down the centre length of a tapered bar?

I know I could measure and mark each end at the centre then just use a straight edge to scribe but would like to find a quicker alternative method.

Red line on image below is where I want to scribe.

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