building upsome wheels dt swiss gives me an actual spoke length and a recomended,why is there several mm inbetween these two and does it matter if i get slightly different sizs by a mm or two
In short, no.
Plug the numbers into [url= http://www.wheelpro.co.uk/spokecalc/ ]spokecalc[/url] or [url= http://www.kstoerz.com/freespoke/ ]freespoke[/url]
And don't use dt's erd's.
I didnt i used my own
You need to be within 2mm of of the calculated length (preferably with a spoke slightly longer than shorter). Ideally you're within 1mm over the calculated size.
The issue is that many spokes are available in 2mm increments, so you sometimes get a recommended length. The more accurate you get the spokes, the less dish compensation you'll need to do when tensioning up, this does make it a lot easier.