Nick - there are a number of things to be wary of with calendar spreads.
Ideally you want there to be an implied volatility (IV) skew between the written leg (in the near month) and the long leg (in a far month), so that you are selling an option with high IV (expensive) and buying an option with lower IV (cheaper).
Remember that the profit from a calendar spread comes from the value left in the long option after the short option has expired - ideally worthless. If you pay too much for the long leg (i.e. if IV is too high and it subsequently falls) then it can have a dramatic effect on the profitability of the position.
I mention this because if you find a position that looks attractive with very high IV in the near month, and lower (but still high in nominal terms) IV in the far month, you need to ask yourself why this is. Very often IV will rise before results and collapse immediately afterwards. It doesn't matter if the price doesn't move very much, the collapse in IV (known as a "volatility crush") will take much, if not all, the potential profit out of the position. Back months invariably have their IV hit after earnings - there will be loads of put buyers going into earnings as an insurance against bad numbers, and once the nos are out the way the IV will come off as the demand for puts dries up.
I think for calendar spreads to work well, IV of the long position should be REALLY low, so that it has nowhere to go. Just because it is low to where it has been is not enough if it is high in nominal terms. Also just because there is a big skew between front and back months doesn't make it a low risk trade. It's the crush on the far month IV that does the damage, and a crush on the near month as well makes little difference. The profit comes from the value left in the far month longs.
HTH