You don't perhaps really need it, but if you strive to get your chain-line to be somewhere in the middle of the cassette it will help;
probably should be around the 48mm mark.
If you put the ring in the middle position it would be better, but I always think it doesn't look right. If you go for say a 113mm length
there is a risk that the crank arms / back of your foot may touch the frame as it will bring DS and NDS inwards by about 5mm. I doubt
a 34T chain ring would start to hit the chain-stay, but it would be good to measure up what distance you can play with.
Favorite quote on Retrobike:
Not worth the petrol to take it to the tip so it's down to the angle grinder to make it small enough to put in the wheely bin.