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: 
I am not saying this is the cleanest or the most practical but I will just tell you what I do... Angle grinder with diamond blade. I cut these pieces only outside and I wear proper goggles and mask. It is one of the only times that I overdo safety. 

This is the only way I can get a clean small cut in HB. If I use a knife it will take a few scores, then I bend it and since the bent area is small, the break won't be even at all.
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.