Normal map baking problems, please help?

I am having problems in baking this normal map from a high poly mesh to a low poly mesh, I am not getting the expected colour gradient. At an extrusion influence of 0.5, the result was horrible but when I increased it to 5 it got better, but the area around the fin remains green which affected the fin I highlighted. I can’t find the right amount of influence to fix this. When I took it up to 10 other parts of the map got messed up.

This is the normal map:

Because the rest looks okay maybe there is just not enoug hmargin at this special UV area so the normal map gets over/wrongly sampled ?

NormalMapProblemt

( There is the not so easy posssibiliy to paint/smooth on the normal map but has to be done in the correct channels (no blue ??) and a normailze function or node has to be used on the new n-map… to correct it… but i muself didn’t do this yet…)

Maybe test (or repair) this with an b/w influence map to not use normals at the fins ??