[R] Predicting X from Y in four-parameter fit (SSfpl)

[Klaus Jensen]

While analyzing titration-data using four parameter fit, I need to
predict single values from the model: I need to predict both ways (ie:
X->Y & Y->X): 

Example:

library(nls) 
cramp<-c(33,100,300,900,2700,8100,24300,72900) 
myo<-c(2.7130,2.6790,1.5255,0.7675,0.3670,0.2150,0.1575,0.1400) 
titration<-data.frame(cramp,myo) 
nls(myo~SSfpl(log(cramp),A,B,xmid,scal),data=titration) 
SSfpl(log(2000), coef(sm)[1],coef(sm)[2],coef(sm)[3],coef(sm)[4]) 

-> [1] 1.060424 

This is an example of predicting single values X->Y 

Here is my question:
How do I predict the other way around Y->X ?? 

Any help is appreciated 

Best regards, 
Klaus 
State Serum Institute, Copenhagen