They are a good tool for progress measuring , but you will need some one to take your readings to get a true reading , then ideally you should use the same individual each time , as individuals vary in there techniques and thus readings, most top bodybuilders avg around 4-5 % body fat at contest time some slightly higher , with dehydration this is a ideal percentage for contest , some get lower , I usually got around 3 % in my caliber readings (but I was renowned for my leanness!!!!) it is a good idea to use them whilst dieting for a show for they indicate to you lean muscle loss ,thus enabling you to adjust calories accordingly to save off being catabolic , the last thing you want whilst getting ready for a show is muscle wastage , kinda defeats the purpose of it all really !! fat loss sure , but muscle ??!!! well why bodybuild , you know what i mean !!!