I don't think there will be a torque value for these bolts as they are screwing into those captured nuts in rubber mountings. You have to do it by feel really in my experience as the rubber does give over time and distort. I used to have a Ducati and all the fairings were attached with these type of fixings and they do come loose every so often. If you over tighten them and the bolt seizes into the nut because of damp or something then it will just turn when you want to undo them. I always put a bit of coppa-slip grease on the threads every time to prevent this. anyway, I wouldn't worry too much - there is no forces applied to the screen really and it will not come off during use.