There is an easy way to create a line cut into render.
"The string line never lies"
You need to establish a reference point and in this photo a measurement is taken from the paving to the top of the sill. That measurement is derived from where the brick is laid on the concrete slab. A set of spring calipers are calibrated to that measurement.
Then based upon your box rule's length strike a small mark in the render with the point of the caliper along the length of the wall.
Hold the box rule on the marks and gently cut the line with a small tool. It is best done at the end of the days work. Don,t dig to deep. Let the render dry overnight.
The next morning you can cut the line. Break a carborundum blade in half and in this case a thin 100mm blade. The quality of the cut in the mortar depends how smooth the cutting edge is. You may have to run the blade along the top of a cored brick to clean up the edge.
When cutting the line try not to break or crumble the edge of the line cut. It may cut perfect first up but if the edges look rough you may have to wait a while.
The line is minimising the cracking of the mortar between the two different substrates.
To mark the wall on a two storey is different altogether. You could margin down from the eaves board and use a bricklayers line and blocks. Because of the scaffold and the varying positions it can be in, will be the mitigating factor whether you can actually do it safely.
Making the same measurement at the ends set the string below the line you want cut. Then make the marks to align the box rule (red line). You can use brickies line pins if the corner blocks are difficult to use. In most cases you will have to put a packer between the string line and wall. This is just to make sure any mortar on the wall does not interfere with the position of the string line.
The most important one is to pull the string line tight. It takes practice by yourself. Once the line is in place, stand at one end and sight along the length of the line, looking to see if it has bow in the middle. If there is a bow you will have to pull the line tighter.
