Your second method is what a lot of guys do. This is similar to a chamber gauge. But bear in mind that your measurement might very well change every time you change bullet weight or manufacturer.
You can then back off .025/.030" and load four or five rounds each in lengths varying .005/.010" to find a sweet spot for that rifle. Then start playing around with different powders,primers,and types of brass.
Hopefully what you end up with, will shoot well in your rifle and still be short enough to fit in your magazine...
