How to measure out correct length for power/fiber/copper cables in a rack?

How do you guys measure out the correct lengths for cabling, when cabling up a new rack?

For CAT5E/6, I used to hand-crimp all my cables, to get the correct length and colour. However, honestly, sometimes it's just easier to buy pre-made.

And for power (C13/C14 cables to the rear PDU), and fiber (single-mode), I nearly always buy pre-made. (I have colleagues that crimp their own power cables, and I do have access to a fusion splicer, but that seems silly for patch cables, right?)

Do you just try the different cable lengths? That seems wasteful....although I used to get a long Ethernet cable and mark metres on it.

Or do you use some kind of measuring tape, or special tool? (If you have specific suggestions let me know).

Or do you calculate it using some kind of rack-planning tool?