diff --git a/focus_app.py b/focus_app.py
index c7aa7f3..03ffd36 100644
--- a/focus_app.py
+++ b/focus_app.py
@@ -125,6 +125,10 @@ def focussing_app(parser, init_seq_func, set_focus_func, get_image_func):
                 pt.with_stem(pt.stem + "_rough")
             )
 
+        # rough estimation: be sure that only a single
+        # acquisition per focus value
+        seq_rough_kwds["acq_per_value"] = 1
+
         result = obsutil.focussingSequence(
             [focus_start, focus_stop],
             init_seq_func,
@@ -141,7 +145,7 @@ def focussing_app(parser, init_seq_func, set_focus_func, get_image_func):
             return result["ret_code"]
 
         # compute precise range start and stop values
-        r = args.num * focus_step / 2.0
+        r = (args.num-1) * focus_step / 2.0
         focus_start = result["focus_value"] - r
         focus_stop = result["focus_value"] + r
 
diff --git a/focus_seq_FLI.py b/focus_seq_FLI.py
index 382c0a2..81b89ab 100755
--- a/focus_seq_FLI.py
+++ b/focus_seq_FLI.py
@@ -21,8 +21,8 @@ import pathlib as pl
 
 
 def init_seq(seq_kwds):
-    # remove extension as Eddy's 'fli_control' add hardcoded '.fit'
-    pt = pl.Path(seq_kwds["root_filename"]).with_suffix("")
+    # replace extension as Eddy's 'fli_control' add hardcoded '.fit'
+    pt = pl.Path(seq_kwds["root_filename"]).with_suffix(".fit")
     seq_kwds["root_filename"] = str(pt)
 
     #
@@ -35,7 +35,13 @@ def init_seq(seq_kwds):
     seq_kwds["start_idx"] = 1
     N = len(foc_files)
     if N > 0:
-        seq_kwds["start_idx"] = N
+        max = 0
+        for fname in foc_files:
+           inum = int(fname.stem.split('_')[1])
+           if inum > max:
+              max = inum
+
+        seq_kwds["start_idx"] = max + 1
 
     return 0
 
diff --git a/focus_utils.py b/focus_utils.py
index 259e9c9..3228bb9 100644
--- a/focus_utils.py
+++ b/focus_utils.py
@@ -335,7 +335,7 @@ def focussingSequence(
         ret_val["ret_code"] = -1
         return ret_val
 
-    N = 7
+    N = 5
 
     if len(focus_range) > 2:
         if np.isclose(focus_range[2], 0):
@@ -421,7 +421,7 @@ def focussingSequence(
         # filename = "{}_{:04d}{}".format(fname, i_foc, file_ext)
         # full_filename = str(pt.Path.joinpath(dir, pt.Path(filename)))
 
-        full_filename = str(rname.with_stem("{}_{:04d}", rname.stem, i_foc))
+        full_filename = str(rname.with_stem("{}_{:04d}".format(rname.stem, i_foc)))
 
         print(log_ident, file=log_output, end="")
         print(