How flat is flat enough?

I'm collecting parts for an 8" dob and I was wondering what type of specs to look for in the secondary mirror. Everyone talks about the wave numbers on the primary, but a bad secondary seems like it could make a good mirror perform average and an average mirror not perform at all. Should I look for 1/20th wave secondary, or is it just not worth it?

The 8" will be f/6 for visual use only. I'm looking at a 1.3" or 1.5" secondary.

thanks,
jerry

Jerry, over the years I have bought secondaries from a host of vendors, and have had many explanations of how accurate they should be. I feel quite confident that some of them I used were good, and some not quite so good, but I personally have never encountered a bad secondary that was obviously bad. Lately I have been buying from either Sky Instruments in Canada or Orion. I believe they may be from the same manufacturer, probably in China.
On balance, I believe the main criteria for a secondary is that what ever figure, or lack thereof, it should be smooth with no bad turning of the edges, either up or down.
FWIW, IMHO !!

Walter