I'm doing this for a fret level and dress and I only have a straight-edge (made of wood) , my problem is they say it its inaccurate if its not notched , now I was thinking why not measure the back of the neck instead ? I did a fret level and now the 1-9 frets are buzzing on the bass side so I might do it again but this time using the back of the neck as a reference for straightness instead of the fretboards frets. What do you think?
I don't think that is a viable method. First, you don't usually want a perfectly straight neck, so just making the back of the neck straight would not be satisfactory. Second, some necks are tapered or shaped so that they are not perfectly straight, so you have no way of removing that variable.

There are a lot of ways of measuring relief - you can use a credit card, or feeler gauges, or even just the pages of a phonebook used like gap gauges will work. Your method does not sound useful to me.
Get a metal straight edge, I would not trust a wood one. I also wouldn't attempt a fret level unless you know what you're doing. Sounds like the first attempt didn't go so well since you ended up with a bunch of buzzing frets.