You were right. According to Microsoft guidelines the 4.6.1 should be enough:
The versions listed here represent the rules that NuGet uses to determine whether a given .NET
Standard library is applicable. While NuGet considers .NET Framework 4.6.1 as supporting .NET
Standard 1.5 through 2.0, there are several issues with consuming .NET Standard libraries that were
built for those versions from .NET Framework 4.6.1 projects. For .NET Framework projects that need
to use such libraries, we recommend that you upgrade the project to target .NET Framework 4.7.2
or higher.
