Hi Sow!
I’m not using any implements to measure, just eye-balling. I sometimes hold my pencil out horizontally in front of me to make sure things are roughly lined-up vertically.
I’m not sure what you mean about blocking in, but there’s lots of good information on the following page about how to start the Bargues:
https://www.smartermarx.com/t/basic-bargue-plate-walkthrough/
Personally, I found Paul Foxton’s website particularly useful.
I hope to see some of your copies on this thread later…!!