Cut your tile to desired length with a straight cut, then cut your 45o, but leave 2mm on the leading edge, as diagram.
Then you can use something like Sait carborundum pads to hand finish before you bring the pieces together.
It will then create a 'birds beak' a V shape if you will.
This gives you a good joint to grout, and it will give it some strength.
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You can get Sait pads from ATS
SAIT Plain Silicon Carbide Discs 125mm | ATS Diamond Tools - http://www.atsdiamondtools.co.uk/node/56