With 75Kg you could almost use just one board (ST126) for that range, but it really depends on the "quality" of the wind at 11kts. If solid 11, you will be ok. Just.
If patchy 11, or you prefer an "easy" (rather than active) style of planing, ST126 may be just a little marginal.
In the lightest conditions, C145+8.5m will have a better ligt wind performance than ST126+8.5m.
Cheers ~ Ian