I haven't done Celtic but have done stained glass (see my avatar! :-D ) I have one of the clover gadgets that makes a scant 1/4" tape, I cut my fabric on the bias. The gadget is good but I saw a link on here a while ago of a way to make tape of any width with a needle 'threaded' through your ironing board. I'll see if I can find it. :-D