This seemed to fix things for me, so it shows some of the areas
needing fixing.
I know that edit_multichoice_form.php is simpler
in validation - I would expect them to be roughly similar, but I haven't
looked seriously to compare ...

nholtz@holtz3:/files/www/data/moodle21/question/type/calculatedmulti$ git diff edit_calculatedmulti_form.php | cat
diff --git a/question/type/calculatedmulti/edit_calculatedmulti_form.php b/question/type/calculatedmulti/edit_calculatedmulti_form.php
index eec57e3..9b32b78 100644
--- a/question/type/calculatedmulti/edit_calculatedmulti_form.php
+++ b/question/type/calculatedmulti/edit_calculatedmulti_form.php
@@ -261,6 +261,8 @@ class qtype_calculatedmulti_edit_form extends question_edit_form {
             }
         }
         if ($data['multichoice'] == 1) {
+            $totalfraction = 0;
+            $maxfraction = 0;
             foreach ($answers as $key => $answer) {
                 $trimmedanswer = trim($answer);
                 if ($trimmedanswer != '' || $answercount == 0) {
@@ -303,8 +305,6 @@ class qtype_calculatedmulti_edit_form extends question_edit_form {
                     $answercount++;
                 }
                 //check grades
-                $totalfraction = 0;
-                $maxfraction = 0;
                 if ($answer != '') {
                     if ($data['fraction'][$key] > 0) {
                         $totalfraction += $data['fraction'][$key];
@@ -324,27 +324,25 @@ class qtype_calculatedmulti_edit_form extends question_edit_form {
 
             /// Perform sanity checks on fractional grades
             if ($data['single']) {
-                if ($maxfraction > 0.999) {
+                if ($maxfraction != 1) {   /// Won't this get caught by totalfraction test, below?  No - this effectively will require one right answer for single choice
                     $maxfraction = $maxfraction * 100;
                     $errors['fraction[0]'] =
                             get_string('errfractionsnomax', 'qtype_multichoice', $maxfraction);
                 }
-            } else {
-                $totalfraction = round($totalfraction, 2);
-                if ($totalfraction != 1) {
-                    $totalfraction = $totalfraction * 100;
-                    $errors['fraction[0]'] =
-                            get_string('errfractionsaddwrong', 'qtype_multichoice', $totalfraction);
-                }
+            }
+            $totalfraction = round($totalfraction, 2);
+            if ($totalfraction != 1) {
+                $totalfraction = $totalfraction * 100;
+                $errors['fraction[0]'] =
+                        get_string('errfractionsaddwrong', 'qtype_multichoice', $totalfraction);
             }
 
             if ($answercount == 0) {
                 $errors['answer[0]'] = get_string('atleastoneanswer', 'qtype_calculated');
             }
-            if ($maxgrade == false) {
-                $errors['fraction[0]'] = get_string('fractionsnomax', 'question');
-            }
-
+///            if ($maxgrade == false) {
+///                $errors['fraction[0]'] = get_string('fractionsnomax', 'question');
+///            }
         }
         return $errors;
     }