I rarely use a builders level for siding it can get you into trouble on older homes especially.
It’s more important that the reveals are the same under and over windows, doors, etc. sometimes that means the siding isn’t actually level although it appears to be.
I like to find a spot where I can pull a string across the entire wall at least a course above the windows. Then I’ll pull a number off the soffits or rafters as a starting point for the string line. Once my string is set I’ll check it to the windows and doors looking for consistent numbers and fudging the string as needed until I’m satisfied. Once that line is set all layout reference comes from that line.
When working your way around the house decide where your ending point is going to be. The best case scenario is if there is a fireplace or some other dead end to finish into so if your line around the house isn’t perfect you will never know.
Another good place to end is a long wall with few windows, if your lines end up off an inch as you came around the house you can make that up on your last long wall since you’ll never see that the siding is out 1” in 30’ but at least the lines will match up.
Just don’t finish the siding on the front of the house, if you do have any problems lining things up you want that to be as far out of site as possible.
