The lower the spring pressure the easier it is to tune the boost control and part throttle.

Use the lowest possible spring pressure that you can, but with which you can still run the desired boost on top.
You can use KFLDRAPP to quickly check this. If you have in high RPM 95% wgdc but boost is still too low (and IAT is ok), then you need to increase the spring pressure.
Of course you need to make sure you don't have any leaks and so on.

Your question can not be answered simply by "tial 38mm". The spring pressure required depends on the backpressure, and that depends on turbo, exhaust, etc. Also how the wastegate has been fitted to the exhaust manifold.
So it is kinda pointless to discuss it. You test and check what is the minimum pressure on the spring that you can get away with and then you use that.

Just aim at ~80-90% wgdc with desired boost


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