Tach mounted kind of

well I attempted to mount my tach on my dash today, placing it over a pre existing crack on the dash , i used 2 lag screws and some double sided tape. it seems to be on there ok the screws never got really tight, i mean it moves with the dash if you push it, but i think youd really have to tug for it to come off, im wondering if i should have drilled all the way through the dash and put a nut underneath, what da ya think, oh well time will tell still needs wiring up heres a pic, you guys tell me