Use a separate error parameter for lower bound.

bench/gen_bench_expectations.py

Index: bench/gen_bench_expectations.py
diff --git a/bench/gen_bench_expectations.py b/bench/gen_bench_expectations.py
index e7d08d116f6d57b1712ef10f71ac4efd18cfa498..6a5ee1e371f55557c53252062e099b669e669d01 100644
--- a/bench/gen_bench_expectations.py
+++ b/bench/gen_bench_expectations.py
@@ -15,7 +15,8 @@ import sys
 RANGE_RATIO_UPPER = 1.2  # Ratio of range for upper bounds.
 RANGE_RATIO_LOWER = 2.0  # Ratio of range for lower bounds.
 ERR_RATIO = 0.08  # Further widens the range by the ratio of average value.
-ERR_ABS = 0.5  # Adds an absolute error margin to cope with very small benches.
+ERR_UB = 0.5  # Adds an absolute upper error to cope with small benches.
+ERR_LB = 1.5
 
 # List of bench configs to monitor. Ignore all other configs.
 CONFIGS_TO_INCLUDE = ['simple_viewport_1000x1000',
@@ -43,8 +44,8 @@ def compute_ranges(benches):
   diff = maximum - minimum
   avg = sum(benches) / len(benches)
 
-  return [minimum - diff * RANGE_RATIO_LOWER - avg * ERR_RATIO - ERR_ABS,
-          maximum + diff * RANGE_RATIO_UPPER + avg * ERR_RATIO + 2 * ERR_ABS]
+  return [minimum - diff * RANGE_RATIO_LOWER - avg * ERR_RATIO - ERR_UB,
+          maximum + diff * RANGE_RATIO_UPPER + avg * ERR_RATIO + ERR_LB]
 
 
 def create_expectations_dict(revision_data_points):