Help please !

Wouldn't this all be relative to the design of the typeface?

See the FLS5 Manual, p.241:
Underline [Position]: This is the position of the middle of the underline line in your font
Thickness: This is the thickness of the underline line.
[They go into the post and CFF table of an OT font.]

As to Strikeout Position and Thickness, these don't seem to be mentioned in the manual, but obviously it's the same for them.
[These go into OS/2 table of an OT font.]

I'd make Underline as thick as the 'underscore' glyph, and Strikeout as thick as 'endash'/'emdash', and position them according to these two reference glyphs. However, I am not aware of applications that actually use these values. Has anybody made some tests?

Karsten