In the end I experimented with some pieces of rubber mat with grass leaf patterns cut into the edge and found that the marks made with the nozzle held close and the mask held tight to the glass were OK. So after blasting in a kind of stepping stone path cut from the original masking tape I stripped off most of the masking and proceeded to put in the last section with a series of these mini masks giving a repeated grassy motif. I learnt a lot during the process with this piece, but I think I would like to do something a bit looser next time and see if a little freedom will teach me some more having got the equipment sorted out a little better than before. The glass sheet is 80x185cm. Looking forward to seeing it in its final location I was disappointed when I took it to the site to find that the wall it was going into hadn't been built yet. So it is in a corner hidden behind a sheet of plaster board until that happens.