I've been playing the guitar for about 40 years so I have major calluses on my finger tips, I find the sides of the fingers near the nail is the best option for me/and the thumbs. As well I noticed there have been times when I can't get any blood out usually because of room temperature and inactivity at the time I do the testing so I'll go for a short run, do a flight of stairs or run a tap of warm/hot water over my hand to get things circulating better. I'm lucky though - just looked at my finger tips now and no scars or bruising