Optimal computer volume?

100% and use the amp for controlling the volume.  If your audio is clipping or you know your computer's sound output is loud [considerably louder than line level], set it to a level closer to line level or until it stops clipping.

If you have an output labled line out use it and set the output to 100%.  If you are using a DAC set it to 100% and you should also be fine with no clipping.

Less than line level then you'll get less dynamic with your music, less impact generally.  But if that's what you're looking for, go ahead and do it.  Some people like more soothing effect, some people like more 'engaging' and dynamic music, you can adjust to your taste.

If I have to amplify from computer headphone out, I usually put it to 100%.

I run my volume at 60% out of the computer and I think it sounds just as good as 100% and less amp power.  Guess it depends on your involved equipment. 

*Ideally* volume should be set at unity gain (generally this is 100%) and have the last piece of hardware at the end of the chain (generally the amp) control volume.

If you set higher than unity gain, you may experience clipping.

If you set lower than unity gain, then there is some digital software volume control is at work which will make you lose resolution. How important this is and whether you can hear it depends on your rig. But it's usually very subtle or unnoticeable unless you have really high end gear and exceptional hearing.

On a side note, I have a Creative X-Mod and I notice effects such as x-fi crystalizer and cmss often push volumes high enough to cause serious clipping at 100% volume. You may be expriencing clipping due to dsp effects on your sound blaster too.

yeah let it be 100 % and after playing music if you feel the voice is distorting, reduce the level to a lower level

not quite 100%: http://www.gearslutz.com/board/tips-techniques/334385-intersample-peaks.html

Using headphone out amped I set computer volume at 100% with no clippin/distortion and use my caliente to control the volume. Seems to me (like others have said) this gives the most dynamic and engaging results. What are you using for playback? I would highly recommend foobar2000 with ASIO4ALL if you are not using that already. Also an external DAC might be something to consider adding to your chain.

Use your ear better, which level you like best

If possible, acquiring a DAC with line-level output to feed into your Little Dot I+ should solve this problem. You should be able to set the software volume controls to 100% without any clipping, and you can rest safe in the knowledge that software volume attenuation isn't impacting on sound quality. Based on my experience, you should hear a noticeable increase in detail using a DAC over the output from your sound card.

I have the Little Dot I+ too and have fed it from a DAC (Little Dot DAC_I) with a fixed line level out, and from a DAC with a variable line-out level (NuForce uDac). With the uDac line out level set to 100% I hear what I believe is a degree of clipping (or distortion anyway), so I have to dial it back to about 70-80% of the maximum volume. With the DAC_I, I don't hear any distortion.

What this comes down to is if you are talking about a Line-level output or an amplified output.

If it's a line-level output, you want it at 100% and you should not experience any clipping with it at 100%

If it's an amplified output, designed to power headphones, etc directly, you would want to set it to less than 100% output but more importantly you'd probably just want to get a source that doesn't suck.  Running through an amplified output is going to be nothing but bad.

Ah. Good spot.