If Y20 is known at the time you decide whether you want > or <, then define a function:
selY20 = @(Var) Var(Var>Y20);
or
selY20 = @(Var) Var(Var<Y20);
depending on which one you want. Then your code would have
T = selY20(fwhm);
If you do not know Y20 at the time you make the decision, then at the time of the decision define a function
selY = @(Var, Y) Var(Var<Y);
or
selY = @(Var, Y) Var(Var>Y);
and code
T = selY(fwhm, Y20);
Or if you want to be more hard-core,
selY = @lt;
or
selY = @gt;
and code
T = fwhm(selY(fwhm, Y20));
A < B is the same internally as lt(A,B) and A > B is the same internally as gt(A,B)
