Just been at the urinals looking at the plumbing pipework. Formed with compression joints and bent pipes.

Is there an easy way of calculating straight pipe lengths to achieve the desired bent pipe lengths/angles etc?

Yes. if its a mandrel bender its typically done on the exact centre line of bend. if its a open formed bend (like a plumbing or hydraulic pipe bender) the theoretical centre line is approx 40% of the OD from the inner bend radius. Exact % is best found through a test piece

Noted - I can probably stop pondering that now!

Hope this post can help you:
https://machinemfg.com/tube-benders-ultimate-guide/