Measure the height from the wall to the top of the pier and then measure from pier to pier. If multiple railings its usually worth sketching it out on a bit of paper then using a mobile phone take a picture of the sketch and email it over.
we use a digital level with a laser to get the required fall, it makes it quick and easy but at over £150 its not something we expect anyone that isn’t in the construction industry to have. But it easy enough to do without the need for expensive levels.
In simple terms for a slopping railing we need the fall in mm per meter. this can be achieved by using a normal level and possibly a plank of wood for a straight edge. Simply put the plank at the top of the slope, pack it up with something suitable until level. now using the level in a vertical position at the end of the plank measure the height from plank to floor. Mark where you measured the height on the slope and on the plank, on the floor then measure along the slope from the top of slope to your marker where you measured the height, also measure the horizontal distance from top of slope to mark on plank. Technically we only need the fall (distance from plank to floor) and the going (horizontal distance from top of slope to mark on plank) once you have these two measurements its easy to work out the fall in millimetres per metre, but the third measurement allows as a balance and check to confirm its correct with the use of Pythagorean Theorem.
Our preferred method on site, is to use a sheet of plywood, lay it on the wall. Mark it out with a pencil, then cut out the shape with a jigsaw. This works exceptionally well as it give us a physical wall profile to match the curve to. You are more than welcome to use our method, the cut boards need to have the top side marked and if multiple parts need to be labelled A, B and C etc, then just drop them off to us at the workshop.
Diy method, is to get a straight piece of timber the desired length (on a large curve made from multiple panels you would use timber the length of the desired panel). Mark out on the timber, lines every 200mm or smaller (ideally justified over the length of the timber, so that the marks are evenly spaced) then lay the timber so the front edge touches the curve both ends. Then starting from the first line mark measure from front edge of timber to the front edge of the wall, repeat along the each mark recording each measurement. Then once we have those measurements we can replicate it to the work bench and join the dots, This should give us a very close representation of the curve, enough to bend the railing to the correct profile.