How much 'hiss' is too much?

-54dB is pretty high as the auto-record kicks in at -40dB, I think; if 3-4 people had ambient noise at -54dB, you're gonna be recorded. On the other hand, I think you'd struggle with an open mic to get better than that: what are you playing or are you singing so you can't DI?

-24dB as an "quiet" average, -18dB as an "in front" average and peaking like you say are good numbers. (Remember to have your sliders set to 0dB in NINJAM when setting up, or you won't be seeing the right levels -- or use pre-NINJAM levels for set-up.)
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...But how do you play with people, for people. Playing fast around the drums is one thing. But to play with people for others, to listen to, that's something else. That's a whole other world. -- Tony Williams

Thanks Peter, I'm kind of "damned if I do, damned if I don't"... if I keep the hiss low, I also get low levels from my playing. I think a large part of it is coming from my laptop fan... but I can't turn it off to find out :P

I can't use a DI if it's any type of acoustic instrument, is that the case?

Well, you can get contact pickups for some acoustic instruments and internal mics for acoustic guitars to get around a lot of environmental noise problems -- no idea on cost or effectiveness (and, from what I've read, they do colour the sound significantly). But if you can't use those, then you'll be using an open mic.

So the next best thing is to make sure you're using the mic to its best advantage in eliminating the noise, by trying to keep the only the source you're after within its pickup pattern. Using figure of eight might let you direct the mic at the source with the laptop in the "dead zone", significantly reducing noise.

Depending on the noise profile, maybe a high pass filter would help.

(As with much of my advice, this is from reading stuff and trying to remember it, not actually doing it..! YMMV :))

Using Filters and a Noise Gate or a Gate which has a Noise Preset will cut most background noise out.
50hz hums can be reduced normally with out effecting the rest of the sound too much.

Im using ReaFir plugin in Substract mode. With FFT Size set to 512 samples there is no audible delay and noise is greatly reduced.

Thanks for the tips guys. I'd tried ReaFir in Subtract mode once before and was unhappy with the results, but I set it up again now and it seems to be working great. I think maybe I had the FFT size set too high previously.

Did I say "So much to learn..." already?

Laptop ? Interesting. .........IJG