If your problem is simply with the unit conversion, you can use em
instead of px
Although em
allows decimal numbers, it doesn't change the rendering accuracy. 0.5px
or equivalent accuracy is not shown on browsers - it'll become either 0 or 1px. Simply because a screen can't show half a pixel, at best it can aproximate that with anti-aliasing.
For smaller fonts the anti-aliasing would probably look worse than the spacing. Another option is finding a web-font that has the spacing you want by default. This way you would get similar results to what you want, but would probably mean changing the font.
The CSS spacing is simply not as accurate as photoshop. One reason is that photoshop renders for printing also - with screen/pixel resolutions you have to live with pixel accuracy. Although theoretically it would be possible with letter spacing that becomes more accurate on zooming etc i don't know of any implementation that would actually work that way.
If correct letter spacing is really important for you, you could try to use SIFR to render the fonts spacing more accurately than what the browser is capable of - they might be able to use changing anti-aliasing. Obviously this would only make sense if the letter-spacing is a big issue.