My problem is form. I can do 230 on machine bench press in five sets of twelve reps, but as you said without the machine to stabilize I become off balance. I did manual for the first time in probably about two years last wednesday and did 185 four times without proper form. It sounds like you have the same problem. Once you get your form down, you could probably put on about thirty pounds easy (bench weight, that is). The point i'm trying to make is that strength ultimately determines how much you can bench, but it isn't everything. Maintaining proper form and a good mental attitude makes up for quite a bit too. Ultimately, having these will make you stronger too.
As for a good amount to bench, it's really up to you. Set a goal for yourself and once reached, raise it up again. It could be in increments of five, ten, or twenty pounds but setting goals keeps you motivated as with anything else. If you can get 135, shoot for 145 the next week. Bench every other day and always have a spotter, I can't stress this enough. Not only does it keep you safe, it helps your form and gains tremendously, especially if all you need is a little help (e.g. the two finger lift). Generally, bencing above your body weight is considered to be good, but don't fret if you can't do this right away. It takes time to get there.
Good luck!