I thought this was standard knowledge amongst plumbers and so on, and therefore amongst DIYers and other plumber wannabees...
Get a hosepipe a bit longer than the distance from one end of your piping run to the other. Fix one end so that the end is exactly at the right height. Go to the other end and fill the hose with water. Once the water starts running out the other end, lower your end a bit and pour a bit more water in. Do this fairly slowly, so you don't get the water sloshing to and from through the hose. Repeat until you can't lower your end without water coming out, and can't pour any more in. At this point, both ends of the hose are exactly level. Mark the wall at this point, and measure relative to that.
If you want to avoid all the lowering and filling, join a bit of clear pipe to the end of your hosepipe and add some food colouring to the water. Fill the hose with colored water, and you'll be able to see the level of the water (which is the level of the far end of the hose if it's full) through the section of clear pipe.
Ob Geek disclaimer: Gravity waves will distort your results.
Ob DIY point: The OGD above is useful when your measurements aren't quite right and water pools in your pipe rather than flowing down it.
Didn't know about that. Cool water level trick. I will remember that.
I just glued my 2m length to my pit. The pipe has been laying in the sun and it bends very well to the shape of the lawn as I discovered when I laid it on the flat deck to cut it and it was still lawn shaped. As I wait for my joint to dry the other 6m bit is laying in place in the trench so it can conform.
I will be glad to get this done, it's a lot of work.