get the bubble close as you can, no need to worry about it too much from there
As I hope you know getting that bubble close depends on the level

Starrett 135A ... should be pretty easy to get bubble close
http://www.starrett.com/metrology/product-detail/135A
Starrett 130 ... maybe a little tougher
http://www.starrett.com/metrology/product-detail/130
Starrett 132 ... tougher yet but still reasonable (0.080"/foot when outside the line)
http://www.starrett.com/metrology/product-detail/132-6
Starrett 98 is the standard pool table mechanic level ... 0.020"/foot when outside the lines
http://www.starrett.com/metrology/product-detail/98-6 (this is the little one)
Starrett 199 ... forget this one :lol: ... about 0.0015"/foot when outside the line

http://www.starrett.com/metrology/product-detail/199Z
Dave <-- has a recently re-vialed (an interesting exercise) 98-6 and a 98-12
<edit> I noticed that you specified the level (and hence the precision) in a later post

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