I had the same issue when I first started using the oal gauge. You just need to go slow and pay real close attention to the feel. You don't want to jam it in, or your measurements will be incorrect. Go easy, and when you just feel resistance, lock it down. Like posted above, measure until you have a consistent measurement, then go with that.
What weapon are you loading for? Are you limited by mag length? My RPR allows much longer loading than what the book says. If your mag allows longer seating, measure from the ogive to the base, and write down your figures for THAT particular bullet. If you switch bullets, you'll need to go through this process again. You may even need to do this when using different lot numbers of the same bullet. I have run into this with some hornady stuff. Document everything and save this info. May come in handy again down the road.