Dial calibers are inexpensive and will get you on mark. Patch thickness might vary but only if the barrel has really deep riflings and narrow lands. Pillow ticking isn't expensive and can get in both 0.015 and 0.018. just take the dial calipers with you and get smallest amount of each. Always check for end of bolt remnants, cheaper.Sadly, not everyone has the proper equipment to measure the bore of firearms. New comers to the world of firearms are the least prepared for such tasks. Even many old timers don't have the right tools.
If you have to bruise your palm on the short starter it's too thick.