I interpreted this as 50% of the roads he rides are steep uphill. Which would be true so long as the other 50% is equally steep downhill.
Going from 46T to 42T in this instance would be a huge difference, maybe overkill depending on how steep we're talking. 42-18 is pretty low. However a 42T chainring may get you into the range you need, then you can experiment with smaller cogs (much cheaper) to find the ideal gear for you. For FG with downhills I would hesitate to go so low that you are in the optimal gear for climbing, one speed is always a compromise.