It’s my understanding that @Jonathjon likes to buy 1km worth of belt with each order. 
I came across this issue back when I was building my Primo. I ended up changing the values and was able to dial it in very quickly using a long steel ruler and cleaning pick.
I decided to make the change because the X & Y values were off by the same amount, and I thought it was related to part and assembly tolerances. Just like feeds and speeds are build specific, I thought steps/mm would be the same.
I probably would have been fine not doing it but I just wanted to minimize the chance for any potential issues down the road.