Equating CG and Tournament difficulty

[Invigilator]

I had been focussing on tournament play for the last few months and am shifting back to cash games. It got me thinking about how buy-ins affect the average level of play one will see and how that tends to compare between cash game and tournament play.

Granted, there are some limiting factors to this comparison:

-tournament and cash game strategies / skill sets are different
-short handed vs full ring strategies / skill sets are different

However, lets assume that we can work around some of these considerations.

The 2 basic questions are: Based on your own experiences

a. What buy-in for an open tournament best equates to the level of play you run into at the following cash game limits?

b. is there a level, where this breaks down? in some cases, I imagine we are dealing with comparitive ranges

0.05/0.10 1-4 dollar buy-in? -levels I have dealt with
0.10/0.25 5-10 dollar buy-in?
0.25/0.50
0.50/1.00
1/2
2/4
5/10
10/20
25/50
50/100