If you wanted to convert from a smith machine max to find a free weight max is there a good ratio or percentage to use as a rough guide?
You can't. The exercises are too dissimilar.
I was testing some of my maxes to see how i compared to the "structural balance" Poliquin talked about in this article:
http://www.t-nation.com/free_online_article/sports_body_training_performance/achieving_structural_balance
But my gym doesn't have a 45 bench so i had to improvise and use the Smith instead. I'll knock off 10kg and use that as a rough benchmark instead.
On the smith machine bench i can do 250 5 times on regular bench i can do 185 5 times so there is a definite difference
Last time i used a smith machine was a lil over a year ago. Squats went up to 405 in the smith and 310 with free weights the first time. Took me about 8 months go get back to 405 with free weights.
Not tried It myself but I think smith vs free weight deadlifts come closer than that. 
CMX - On the smith machine bench i can do 250 5 times on regular bench i can do 185 5 times so there is a definite difference
yeah, i had a similar difference when iw as younger....maybe 60lb change? i was able to build back up to it on freewights, though...
to the OP, why do you need to convert it? just use the figure you have for your logs and call it good... estimating the difference doesn't really matter, and is prolly not that acurate.
Where u at, HULC, Planet Fitness?
The gym is just an addition to the gym space where i train wrestling and Judo at the moment. One flat bench, One Squat Stand, a few machines, a Smith, some treadmills, One adjustable bench, and a gaggle of dumb bells. It's basic, but i quite like the place.
I used to have a complementary membership of one of the best gyms in London but hardly ever used the place. A wasted opportunity looking back on it.