library(drc)

response <- c(95.23910, 100.23175, 104.52915,  83.71767,  84.91520,  84.99668,  83.75922,  80.79488,  79.44675,  77.64417,
                  77.03753,  77.32093,  74.83431,  80.04922,  83.98284, 100.08766, 100.29243,  99.61991,  98.00136,  97.55210,
                  96.64827,  70.22343,  81.60977,  72.12828,  98.24016, 102.15456,  99.60528,  81.56161,  77.09402,  73.84495,
                  82.32386,  72.07693,  72.32379,  73.78001,  74.79628,  76.88044)

    dose <- c(0.00000,    0.00000,    0.00000,  212.28178,  204.11203,  216.96099,  435.78343,  435.18381,  432.55421,
              1170.87791, 1176.20623, 1154.72914, 2846.47703, 2827.86970, 2797.45003,    0.00000,    0.00000,    0.00000,
              84.85290,   86.36662,   86.55263, 2568.00710, 2487.90320, 2495.58007,    0.00000,    0.00000,   0.00000,
              321.52090,  492.90972,  607.62896,  737.72523,  812.55645, 1232.82452, 1222.24212, 1451.51058, 1728.99578)

    data <- data.frame(dose, response)

    #Attempt 1 - changing the ED function
    Pb <- drm(response ~ dose, data = data, fct = LL.4(fixed=c(NA,NA,NA,NA),names=c("Slope", "Lower", "Upper", "ED50"))) #4 param log logistic model
    summary (Pb) #upper limit = 100 lower limit = 76
    plot(Pb)
    ED(Pb,c(10,50), interval="delta") #relative ED value calculated between upper and lower limits
    ED(Pb,c(10,50), interval="delta", type=c("absolute")) #setting type to absolute = NaNs produced
    ED(Pb,c(10,50), interval="delta", lref = 0) #no change with lref = 0
    ED(Pb,c(10,50), interval="delta", lref = 0,type=c("absolute")) #NaNs produced
    ED(Pb,c(10,50), interval="delta", bound = TRUE) #no change with Bound = True

    #Attempt 2 - changing the LL.4 function
    Pb.2 <- drm(response ~ dose, data = data, fct = LL.4(fixed=c(NA,0,NA,NA), names=c("Slope", "Lower", "Upper", "ED50")))
    summary (Pb.2) #upper limit = 100 lower limit = 76
    plot(Pb)
    ED(Pb,c(10,50), interval="delta") #no change
    ED(Pb,c(10,50), interval="delta", type=c("absolute")) #NaNs produced
    ED(Pb,c(10,50), interval="delta", bound = TRUE) #no change

100 - desired Inhibition value

Pb.2 <- drm(response ~ dose, data = data, fct = LL.4(fixed=c(NA,NA,100,NA),names=c("Slope", "Lower", "Upper", "ED50"))) #I've fixed the upper to 100%
ED(Pb.2,c(85), interval="delta", type="absolute")

Estimated effective doses
   Estimate Std. Error   Lower   Upper
e:1:85  223.048     30.218 161.569 284.528

plot(Pb.2)
abline(h=85)
abline(v=223)