glmfit: deriving x values from the fit and their confidence interval

I am using glmfit (MATLAB 7.11.0 R2010b on a Windows 7 64-bit PC) to fit performance data from a psychophysical experiment. Here x represents differing temporal intervals between a beep and a flash being presented to the subjects, y represents the number of "beep first" responses for each temporal interval, and n represents the total number of responses for each interval.

E.g.

x = [-200; -150; -100; -50;  0; 50; 100; 150; 200];
y = [  20;   17;   15;  11;  8;  7;   4;   0;   1];
n = [  20;   18;   19;  20; 20; 17;  20;  20;  19];

b=glmfit(x,[y n],'binomial','link','logit');
yhat=glmval(b,x,'logit');
plot(x,y./n,'o',x,yhat,'-');

I am interested in determining the value of x for a pre-defined value of y/n (e.g. 75% performance), as well as confidence intervals around that x value. Is there a way to do that?

Thanks,

Pierre Megevand