Question: When laying hardie backer I was told to make my joints as tight as possible, so I imagine I'll run into this often.

Currently I have a standard 6' board that just barely doesn't fit inside the lip of the tub (I'm leaving a 1/4" gap below per hardie board's instructions).

What is the best way to shave off a piece of hardy board so I can make a tight fit?
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Answer: 
You can use one of those cheese grater looking rasps most people use for drywall. They're called serrated contour planes.

Something like the [Stanley Surform Pocket Plane](http://www.stanleytools.com/default.asp?CATEGORY=SURFORM%20TOOLS%20AND%20BLADES&TYPE=PRODUCT&PARTNUMBER=21-399&SDesc=6"%20Surform®%20Pocket%20Plane) or [Stanley Surform Plane-Type](http://www.stanleytools.com/default.asp?CATEGORY=SURFORM+TOOLS+AND+BLADES&TYPE=PRODUCT&PARTNUMBER=21-296&SDesc=Surform%26%23174%3B+Plane+Type+%96+Regular+Cut+Blade)

Those tools will be quicker/easier and make less of a mess than using a power saw. Or at least a more controlled mess. You can also use a utility knife with a sharp blade but the rasps are easier to use and safer.
we used a tub saw with an old blade.(NO WATER) cut out doors. use face mask and eye protection as it makes clouds of dust. draw size and cut free hand a the guide seemed to bind. cuts like butter!! even a perfect bevel.