How do I Cut hair at a long fixed length?

I haven't read much into this site, but I didn't readily see a guide or a tool describing how to cut long hair at a fixed length with ease?

I have a Norelco ProCut hair trimmer which is easy for cutting short hair (less then 2 inches) with the attachment comb, but now that my hair on the top of my head has grown beyond 4 inches in length, I no longer can use the attachment comb. Norelco as far as I know doesn't sell attachment combs for my length of hair.

Using scissors for cutting fixed length hair isn't easy for me because there is nothing guiding the scissors to cut at fixed length (which is why the attachment comb was so easy) without being inexact trying to measure length with my hands.

If anyone knows of a easy way to add more length to my attachment comb of my hair trimmer I'd be delighted to know, or if there is a easy trick behind cutting long hair at fixed lengths without the need for such attachments.

Chris :o)