Hi Andy,
The density of the unit should match the density of your own side and back hair. I've seen guys in their 20's with very thin sides and guys in their 60's with very thick sides. So the guy in his 20's would order a thinner density than the older guy would order a thicker density. The hair style also determines the density as longer hair adds more fullness to a unit. If you'd like to send me a photo (or post it here on the forum) of your back and sides I can better advise you on density. Remember 65% is medium density.
Regards,
John