Thanks for all the hints. The grid sounds like a good idea but it's also the most expensive option - and my fridge has just carked it
. What I like about it is that it can be used hand-held or on a tripod. An electronic level can be used hand-held but the need for more batteries and more lights and buzzes doesn't appeal. The bubble level is nice in that it's simple and low-tech. But I really like the idea of using the focussing marks. I thinks I'm going to try that first and see how it goes.