diff --git a/pytest_split_tests/__init__.py b/pytest_split_tests/__init__.py
index 9658bdd..f224b51 100644
--- a/pytest_split_tests/__init__.py
+++ b/pytest_split_tests/__init__.py
@@ -6,21 +6,23 @@
 from _pytest.config import create_terminal_writer
 import pytest
 
+def get_group_size_and_start(total_items, total_groups, group_id):
+    """Calculate group size and start index."""
+    base_size = total_items // total_groups
+    rem = total_items % total_groups
 
-def get_group_size(total_items, total_groups):
-    """Return the group size."""
-    return int(math.ceil(float(total_items) / total_groups))
+    start = base_size * (group_id - 1) + min(group_id - 1, rem)
+    size = base_size + 1 if group_id <= rem  else base_size
 
+    return (start, size)
 
-def get_group(items, group_size, group_id):
+def get_group(items, total_groups, group_id):
     """Get the items from the passed in group based on group size."""
-    start = group_size * (group_id - 1)
-    end = start + group_size
-
-    if start >= len(items) or start < 0:
+    if not 0 < group_id <= total_groups:
         raise ValueError("Invalid test-group argument")
 
-    return items[start:end]
+    start, size = get_group_size_and_start(len(items), total_groups, group_id)
+    return items[start:start+size]
 
 
 def pytest_addoption(parser):
@@ -40,8 +42,8 @@ def pytest_collection_modifyitems(session, config, items):
     yield
     group_count = config.getoption('test-group-count')
     group_id = config.getoption('test-group')
-    seed = config.getoption('random-seed', False)
-    prescheduled_path = config.getoption('prescheduled', None)
+    seed = config.getoption('random-seed')
+    prescheduled_path = config.getoption('prescheduled')
 
     if not group_count or not group_id:
         return
@@ -70,14 +72,13 @@ def pytest_collection_modifyitems(session, config, items):
                           if test_name in test_dict]
     unscheduled_tests = [item for item in items if item not in all_prescheduled_tests]
 
-    if seed is not False:
+    if seed is not None:
         seeded = Random(seed)
         seeded.shuffle(unscheduled_tests)
 
     total_unscheduled_items = len(unscheduled_tests)
 
-    group_size = get_group_size(total_unscheduled_items, group_count)
-    tests_in_group = get_group(unscheduled_tests, group_size, group_id)
+    tests_in_group = get_group(unscheduled_tests, group_count, group_id)
     items[:] = tests_in_group + prescheduled_tests
 
     items.sort(key=original_order.__getitem__)
diff --git a/tests/test_groups.py b/tests/test_groups.py
index 8cb9383..8f5a1e9 100644
--- a/tests/test_groups.py
+++ b/tests/test_groups.py
@@ -1,25 +1,25 @@
 import pytest
 
-from pytest_split_tests import get_group, get_group_size
+from pytest_split_tests import get_group, get_group_size_and_start
 
 
-def test_group_size_computed_correctly_for_even_group():
-    expected = 8
-    actual = get_group_size(32, 4)  # 32 total tests; 4 groups
+def test_group_params_computed_correctly_for_even_group():
+    expected = [(0, 8), (8, 8), (16, 8), (24, 8)]
+    actual = [get_group_size_and_start(32, 4, group_id) for group_id in range(1, 5)]  # 32 total tests; 4 groups
 
     assert expected == actual
 
 
 def test_group_size_computed_correctly_for_odd_group():
-    expected = 8
-    actual = get_group_size(31, 4)  # 31 total tests; 4 groups
+    expected = [(0, 8), (8, 8), (16, 8), (24, 7)]
+    actual = [get_group_size_and_start(31, 4, group_id) for group_id in range(1, 5)]  # 32 total tests; 4 groups
 
     assert expected == actual
 
 
 def test_group_is_the_proper_size():
     items = [str(i) for i in range(32)]
-    group = get_group(items, 8, 1)
+    group = get_group(items, 4, 1)
 
     assert len(group) == 8
 
@@ -27,7 +27,7 @@ def test_group_is_the_proper_size():
 def test_all_groups_together_form_original_set_of_tests():
     items = [str(i) for i in range(32)]
 
-    groups = [get_group(items, 8, i) for i in range(1, 5)]
+    groups = [get_group(items, 4, i) for i in range(1, 5)]
 
     combined = []
     for group in groups:
@@ -40,11 +40,11 @@ def test_group_that_is_too_high_raises_value_error():
     items = [str(i) for i in range(32)]
 
     with pytest.raises(ValueError):
-        get_group(items, 8, 5)
+        get_group(items, 4, 5)
 
 
 def test_group_that_is_too_low_raises_value_error():
     items = [str(i) for i in range(32)]
 
     with pytest.raises(ValueError):
-        get_group(items, 8, 0)
+        get_group(items, 4, 0)