Hey there! You should be able to adjust the specular value by changing the decal blend mode in the material:
The blends modes of Translucent and Stain should both be able to accept a Specular value for you to adjust and match. Some of these blend modes don’t appear to be 100% as expect though, as setting it to Color only still creates a spec value of 1, while Normal Only seems to correctly only effect normals, odd. Hopefully you can get the correct effect you are desired by changing to Translucent Blend mode.