I have measured new chains before, by going round both biggest cogs and adding two links, always worked great, but am doing a new build and using the Shimano saint shadow rear derailleur, when I have done the above measuring. When going through the derailleur and on small cogs, chain is way to long? Any ideas please.?

I always just do it “right” by which I mean, put it in the biggest combo of cogs, then cycle the suspension. That way you can find the tightest possible spot and adjust accordingly (and you might learn something interesting, like “If I adjust the chain so that all the gears work at the tightest possible spot, the chain is too slack”

The Saint has a titchy wee cage so doesn’t pull as much chain as most mechs