I am doing a wedding cake this week using basketweave with a 2B tip. How do I measure the spacing on the tiers? 2 will be square and 2 will be round. Does that make a difference?
Try reading through this thing from Wilton. I believe the answer to your question is in step 2.
http://www.wilton.com/technique/Basketweave
Hope this helps. 
I usually turn my tip side ways and lightly press it into the the icing to give me the width of space to skip.
If you have the Wilton flower formers you can also use those pressed into the cake to make evenly spaced lines to follow.
For a the smaller basket weave tip I use a small flexable metal spatula to get the size inbetween the vertical lines. I like TexasSugar's idea of using the flower formers and the width of the tip for the bigger tip.
For the Horizontal Spacing I just place the tip on the icing, as if I was going to create a line right above the previous one, to leave a small bit of icing or an indentation. This way you have a the exact spaceing that the tip will fill on the next row.
I use the same technique for a round and square cake except you have to wrap the basketweave around the corner so you want to try to get the horizontal line on the corner.
Hope that helps.
Thanks to all the response. 
I guess I should have made it clear that I was concerned about the vertical spacing and it coming out even. The 2B tip is wider than the one used in class and I'm sure there is a simple formula to figure the width apart each line should be marked. I may just be stressing over miniscule details since this is only my 3rd wedding cake and the most detailed so far. However, I am greatful for everyone here because I know that if you ask someone knows the answer and they are always so nice about it. I have learned so much from this site.
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