To Lime or Not to Lime ??

Never lime where potatoes are going, it causes scab.

I only lime where my brassicas are going, as crops rotate around there is enough to last til the brassicas come back to that place.
If you really want to be precise you could get a soil testing kit.

John's advice is here  http://www.allotment-garden.org/composts-fertilisers/lime-the-vital-fertiliser/